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rheinsberg2025-06-03 03:54 pm
aella_irene) wrote in rheinsberg2025-06-03 03:54 pm
Counterfactual: The Tale of Friedrich Karl Emil
It began with me being unsupervised at the Charlottenburg Palace, and posting "I'm in Berlin, and I find myself wondering...if Fritz and EC had somehow conceived a child during the Rheinsberg years, what happens? Other than trauma."
selenak
Good lord. Definitely trauma. The best case scenario I can see is that it‘s a girl, because generally Fritz (despite the misogyny) did better with his female relations. In rl, his niece had good memories of him, and his younger sisters got along better with him than his younger brothers, which is a sort of precedent for this AU. Also, if he has a daughter, he won‘t feel threatened by her or want her to be exactly like him or would resent her if she is like him. If this daughter has some musical talents and likes literature, he might be tempted to make her into the second Wilhelmine, and if she doesn‘t and is more like EC, he still won‘t be that disappointed because of the general low opinion of women. EC will most likely be happy and love a daughter as well, and at any rate will feel more like there was a point to her existence.
If it‘s a boy, then FW will feel very happy and justified and Fritz will start with the resentment on that count alone. (EC will be very happy, and the angst won‘t start until the kid starts to talk.) He might still want a childhood for the kid that is unlike his own, but unfortunately unless the kid is a very bright toddler that will probably result in what happened with real life nephew FW2, who got all the teachers and lessons Fritz couldn‘t (openly) have because of FW‘s own issues with learning and was completely overwhelmed. Now Fritz did love the younger of his nephews, Henricus Minor, so there is a positive rl example as well as a negative, but unfortunately we don‘t know much about Henricus Minor due to his dying so young. I would guess that if the hypothetical son of Fritz and EC is shy - like rl FW2 was - we get the worst case scenario of the kid being given the same treatment (i.e. Fritz orders the teachers to tease and bully him out of the unbecoming shyness), only multiplied because it‘s his own son. If the kid is more the kind of extrovert adorable imp that apparently both AW and (at a guess) Henricus Minor were, then once FW has died and can‘t play the proud grandfather who alwys knew what the wretched son needed was a proper wife and kids anymore, there might be a shot for a more positive than negative father-son relationship because Fritz responded well to the outgoing affectionate and gregarious types. But if the kid is more an introvert, let alone shy: DOOOOOOM. TRAUMA.
AW: If Fritz has a son, does that mean I can divorce Luise and marry the love of my life? (Whoever she might be, either Sophie or Mina.)
Fritz: NO.
Heinrich: If a son of Fritz is the Crown Prince and I thus will not be able to hope for a future where I am AW‘s consigliere, I will see my existence pre 7 Years War as even more pointless and will probably be even more rebellious. Unless I‘m not forced to marry because Fritz is busy roleplaying Dad and himself with his own kid instead of me?
Fritz: Don‘t count on it.
MT: I pity any offspring this man might have. However, I suddenly see some potential for a return of Silesia a generation later if the kid is properly influenced. Maybe more. Hm. Which of my daughters might make a good Queen in Prussia?
Me: Because I love angst, I immediately wonder about a girl (Wilhelmine Again?) and a shy boy, whose sister has inherited the Fritzian genius, and also a desire to defend him from their father, while EC is torn between her affection for Fritz and a need to protect her children.
Heinrich: why am I now responsible for this seven year old on a battlefield?
I am also wondering, assuming that no way no how would Fritz have a daughter as heir, if he would go for AW and his kids, or for his daughter's children. AW is born 1722, while Hypothetical Daughter is born between 1733 and 1740, so marrying her to one of AW's kids would come with a potentially problematic age gap.
selenak:
It's not just a question of "Wouldn't", he literary couldn't. Salic law and all that applying to German principalities. (Note MT is not, repeat, not, technically Empress Regnant of the HRE, she's the Empresss Consort once FS has become Emperor. What she is undisputed Queen Regnant of is Hungary and Bohemia, where Salic Law does not apply.) (Hence when we speculate how Catherine the Great would have fared if she hadn't married (P)Russian Pete but had remained Sophie von Anhalt Zerbst and married Heinrich instead, there's no scenario where she could have done what she did in rl and end up as single Queen Regnant. (Regent, yes. Queen Consort, yes. Queen Regnant, no.)
I think while going for hypothetical daughter's children would have been possible if there were no alternative male heirs, the fact that Fritz not only had three living brothers but that two of them (AW and Ferdinand) had themselves sons, not to mention that his Great Grandfather, the Great Elector, had spawned a whole separate line of Hohenzollern in his last marriage (the Schwedt cousins, one of whom Bach wrote the Brandenburg Concertos for) would have meant such a scenario need not apply. Besides, and speaking of the Schwedt cousins, another of those married in RL Fritz' sister Sophie. Presumably he could have married a daughter to a Schwedt cousin instead in that alternate timeline in order to reunite the two Hohenzollern branches and avoid any foreign prince getting to be King in Prussia.
(Not 100% sure he'd have done it if only because he didn't have a very high opinion of the Schwedt cousins, but it would have made 18th century dynastic sense.)
Me:
I always get confused about where Salic law does/does not apply, and went for 'doesn't' here because of Anna of Prussia--and the assumption that the HRE was an elective oddity.
Well, probably for the best that Fritz can't try and create a perfect grandson via his daughter, as the daughter tries to take MT as a role model. And I think she'd be the wrong age to marry Paul of Russia, however much he might like the idea.
mildred_of_midgard:
I'm reminded that in the 13th century, a Hohenzollern Swabian Duke got permission from Rudolf of Habsburg that a woman could inherit if the male line died out...but I agree that nobody would see having 3 brothers and however many nephews as the male line dying out
Back at the originalselenak comment...
mildred_of_midgard:
Ooh, this is bad. Very bad. :P Poor kid, whoever he/she was.
I agree with everything you say here, Selena.
the kind of extrovert adorable imp that apparently both AW and (at a guess) Henricus Minor were
Yeah, and personality aside, I'm betting Henricus Minor benefitted from not having all the weight of expectations of being the heir plus being seen as a potential threat.
I agree with your guess that he was an extravert adorable imp, because Fritz's Very Bad Condolence Letter to Ulrike on HM's death said that HM was a lot like his father, minus his father's shortcomings (omg, Fritz).
MT: I pity any offspring this man might have. However, I suddenly see some potential for a return of Silesia a generation later if the kid is properly influenced. Maybe more. Hm. Which of my daughters might make a good Queen in Prussia?
Lololol! Tu felix Austria nube!
So which of the daughters *would* she choose as queen in Prussia?
Or, conversely, who does Fritz's son marry, if not one of MT's daughters?
cahn
My first kneejerk response was "OMG that poor kid!!" :/
Also, if he has a daughter, he won‘t feel threatened by her or want her to be exactly like him or would resent her if she is like him. If this daughter has some musical talents and likes literature, he might be tempted to make her into the second Wilhelmine, and if she doesn‘t and is more like EC, he still won‘t be that disappointed because of the general low opinion of women.
Oh man, I never even thought of the possibility a kid of Fritz's could not have a terrible time, but... this sounds plausible. Ironic that it's because of misogyny!
But if the kid is more an introvert, let alone shy: DOOOOOOM. TRAUMA.
:(((((((((((((
I also imagine in my head the boy being more like FW in tastes and wanting, like, less French and more cannons, and Fritz being very awful to him in the FW-reverse way, because how DARE this kid not appreciate all the things that he would have loved to have!
AW: If Fritz has a son, does that mean I can divorce Luise and marry the love of my life? (Whoever she might be, either Sophie or Mina.)
Fritz: NO.
AW, Heinrich had to marry and it sure wasn't for the succession!
Me:
I return, properly logged in (I didn't take my laptop to Berlin), with the urge to give these unfortunate hypotheticals names. My instinct is to go for a daughter b. 1737 and a son b. 1739 (after, I think, FW1 has sent them a velvet bed?) and I fear that there is no chance this unfortunate son doesn't get forced to be another FW, as the other name option from FW1's ancestry is George. The daughter is probably another Sophia, and I feel like Frederick would fight for a Wilhelmine.
selenak
Now that's slander. ;) Going by the names FW gave his own kids, including the son he had before Fritz (and even before Wilhelmine) died as babies, he was entirely willing to combine "Friedrich" with "Ludwig" instead. So a grandson could have been called "Friedrich Ludwig" as well. FW naming the younger sons after his pal and competition from Saxony (the August in both August Wilhelm and August Ferdinand) would indicate a readiness to honor a living neigbour as well, and given this hypothetical kid would be half Braunschweig, he might go for "Friedrich Karl" or "Friedrich Ferdinand" (both Brunswick names).
(If Fritz gets to choose at all, I'm tempted to go with "Karl Emil", which is the name he suggested for the baby his sister-in-law Luise was pregnant with when AW died. Back in the day we thought that was random, but since then we found out Karl Emil was in fact the name of the Great Elector's original oldest son, big brother to future F1, who died young, and thus already a Hohenzollern name.)
The daughter is probably another Sophia, and I feel like Frederick would fight for a Wilhelmine.
Agreed on both counts.
If there's a son born in 1739 specifically, btw, it might cause some interesting developments. Because that year the father/son relationship between FW and Fritz took another sudden downturn in tandem with FW's health, so much so that according to the gossipy envoys and according to the Fritz and Wilhelmine letters, FW even made noises about the succession. Now he couldn't have altered the succession from Fritz to AW, no matter what he threatened, without Imperial backup and the Diet of Regensburg agreeing. But if he suddenly has his first grandson in the direct line of descent, not by a daughter, then either he's so pleased that Fritz is spared a few paternal rants, OR he makes noises about "what if I alter the succession so your son inherits directly from me, provided I stay alive for the next two decades as well?", which would be a bit easier to do, I suspect (though I guess would still need Imperial backup.)
Me: I apologise to FW most heartily! I also note that we are entering the period of three given names, so possibly Elisabeth Christine manages to get a Sophia Wilhelmine Elisabeth, or Sophia Wilhelmine Antoinette, in there. And this, of course, adds additional options for little baby Friedrich Karl Emil.
Given FW's...FW, possibly he starts off by sparing the rants, then goes to "what if I create him my heir? What THEN?" and then dies before he can change the succession, and submit Prussia to the rule of an infant who cannot yet consume solid food. (For added irony, possibly FKE either dies before Fritz, or survives him only a little, so rule by infant or toddler is only put off for a generation.)
So, here they are, Sophia Wilhelmine Antoinette of Prussia, b. 1737, and her little brother Friedrich Karl Emil, b. 1739, ready for the horrors of the 18th century. Specifically...marriage.
I wanna marry Friedrich Karl Emil
This is an aggregation (and amendment) of several of my comments about who would marry hypothetical-FKE. This assumes that FKE is born in 1739, and marries in 1765, aged 26.
Catholics
Holy Roman Empire! The clear option here is Maria Elisabeth of Austria, b. 1743. Historically, she never married, and was Abbess of the Theresian Institute of Noble Ladies, but she was certainly considered and negotiated for, they just never found anyone they liked, and then she caught smallpox, which scarred her for life. According to Wikipedia "In adolescence, Elisabeth was described as unstable and eccentric, without any particular interests. She was fun-loving and neglected her studies. She earned her mother's censure by liking gossip and mocking everyone, offending her siblings and attendants with ironic comments. Her staff changed often, as few could stand her. Elisabeth was a difficult child to raise, and five ayas were appointed in turn to educate her; even her confessor needed to be changed once. Her mother considered her childish and immature, calling her eine Kokette der Schönheit ('a pretty coquette'), observing: 'It mattered not if the look of admiration came from a prince or a Swiss guard, Elisabeth was satisfied.'" Would this put Fritz off, or not more than 'having MT for a mother and being Catholic'?
Poland-Saxony. Maria Kunigunda of Saxony b. 1740! Historically failed to marry after she was too shy for Joseph II, and everyone assumed she had something wrong with her. Lucky escape, says I. Instead, she became an Abbess, not for vocation, but for lack of other options. She was the youngest of sixteen, and while two of her elder sisters remained unmarried, the others became Dauphine, Queen of Bavaria and Queen of Spain. While religion remains an obstacle, her father converted to get Poland, so converting back might be easier. As an Abbess, she was apparently an excellent businesswoman, and also benefited from her parents' interest in their children's education.
Bavaria. Maria Josepha of Bavaria, b. 1739. Could it be worse than marrying Joseph II? It would also help Fritz in the War of the Bavarian succession, which doesn't happen until the 1780s, but is probably visibly in the offing.
Protestants
United Kingdom. There are a couple of options here, but I don't think Fritz would or should go for them. In the early 1760s we have three unmarried daughters, Augusta (b. 1737), Louisa (b. 1749) and Caroline Matilda (b. 1751.) Louisa has tuberculosis, and her marriage negotiations came to nothing because of her health issues. Caroline Matilda is 13, and might be a possibility. Augusta, meanwhile, would be a better possibility, but historically, of her four sons, three had significant mental and physical health issues, with two of them being excluded from the line of succession, and it seems unkind to put that possibility into the Prussian court. She also greatly disliked anywhere but England, and took out her bad marriage on her daughter Caroline. Obviously, none of this can be known at the time, but still...
Mecklenberg-Strelitz. Christiane, b. 1735, received a comprehensive education, but never married. Elder sister of Queen Charlotte of the United Kingdom, which might be a benefit. On the older side.
Brandenburg-Bayreuth. Elisabeth Friederike Sophie, b. 1732. Historically married in 1748, so is not really still available by the time her little cousin reaches adulthood, but I could see Fritz arranging the match much earlier because Wilhelmine's daughter.
Brunswick-Wolfenbuttel. The daughters of Fritz's sister! Sophie Caroline Marie, b. 1737. Anna Amalia b. 1739. Elisabeth Christine, b. 1746. Very little to choose between them, and of course Elisabeth Christine did marry FW2, but with consequences I doubt could be averted with a shyer, more introverted husband...
Brandenburg-Schwedt. Daughters of another sister! Friederike b. 1736, Anna Elisabeth Louise b. 1738 and Philippine b. 1745. Historically, Anna married her uncle Ferdinand in 1755 and they had seven children.
Denmark. Sophia Magdalena of Denmark b. 1746, but she was betrothed to Gustav III of Sweden aged 5 by the Riksdag of the Estates. Her younger sisters Wilhelmina b. 1747 and Louise b. 1750 have nothing against them, and would get Denmark that Prussian alliance that Sweden has...
Sweden itself hasn't got a princess until 1753.
What are people's thoughts? Who, and what important notes, have I missed? Am I too visibly biased in favour of Maria Kunigunda, except for her name?
Whether SWA marries, I could not say. I feel like either Fritz finds her the most brilliant match possible, or her makes her a lady Abbess so that she can stay with him like Wilhelmine couldn't, and she gets to have an affair with her SIL.
(I also like the idea that, whatever FKE's personality was, he has the reproductive luck of the father of the current King of Sweden, who had four daughters before they managed a son, and died nine months later)
Heinrich, in 1786, made regent for a small child: My time has come.
selenakGood lord. Definitely trauma. The best case scenario I can see is that it‘s a girl, because generally Fritz (despite the misogyny) did better with his female relations. In rl, his niece had good memories of him, and his younger sisters got along better with him than his younger brothers, which is a sort of precedent for this AU. Also, if he has a daughter, he won‘t feel threatened by her or want her to be exactly like him or would resent her if she is like him. If this daughter has some musical talents and likes literature, he might be tempted to make her into the second Wilhelmine, and if she doesn‘t and is more like EC, he still won‘t be that disappointed because of the general low opinion of women. EC will most likely be happy and love a daughter as well, and at any rate will feel more like there was a point to her existence.
If it‘s a boy, then FW will feel very happy and justified and Fritz will start with the resentment on that count alone. (EC will be very happy, and the angst won‘t start until the kid starts to talk.) He might still want a childhood for the kid that is unlike his own, but unfortunately unless the kid is a very bright toddler that will probably result in what happened with real life nephew FW2, who got all the teachers and lessons Fritz couldn‘t (openly) have because of FW‘s own issues with learning and was completely overwhelmed. Now Fritz did love the younger of his nephews, Henricus Minor, so there is a positive rl example as well as a negative, but unfortunately we don‘t know much about Henricus Minor due to his dying so young. I would guess that if the hypothetical son of Fritz and EC is shy - like rl FW2 was - we get the worst case scenario of the kid being given the same treatment (i.e. Fritz orders the teachers to tease and bully him out of the unbecoming shyness), only multiplied because it‘s his own son. If the kid is more the kind of extrovert adorable imp that apparently both AW and (at a guess) Henricus Minor were, then once FW has died and can‘t play the proud grandfather who alwys knew what the wretched son needed was a proper wife and kids anymore, there might be a shot for a more positive than negative father-son relationship because Fritz responded well to the outgoing affectionate and gregarious types. But if the kid is more an introvert, let alone shy: DOOOOOOM. TRAUMA.
AW: If Fritz has a son, does that mean I can divorce Luise and marry the love of my life? (Whoever she might be, either Sophie or Mina.)
Fritz: NO.
Heinrich: If a son of Fritz is the Crown Prince and I thus will not be able to hope for a future where I am AW‘s consigliere, I will see my existence pre 7 Years War as even more pointless and will probably be even more rebellious. Unless I‘m not forced to marry because Fritz is busy roleplaying Dad and himself with his own kid instead of me?
Fritz: Don‘t count on it.
MT: I pity any offspring this man might have. However, I suddenly see some potential for a return of Silesia a generation later if the kid is properly influenced. Maybe more. Hm. Which of my daughters might make a good Queen in Prussia?
Me: Because I love angst, I immediately wonder about a girl (Wilhelmine Again?) and a shy boy, whose sister has inherited the Fritzian genius, and also a desire to defend him from their father, while EC is torn between her affection for Fritz and a need to protect her children.
Heinrich: why am I now responsible for this seven year old on a battlefield?
I am also wondering, assuming that no way no how would Fritz have a daughter as heir, if he would go for AW and his kids, or for his daughter's children. AW is born 1722, while Hypothetical Daughter is born between 1733 and 1740, so marrying her to one of AW's kids would come with a potentially problematic age gap.
selenak:It's not just a question of "Wouldn't", he literary couldn't. Salic law and all that applying to German principalities. (Note MT is not, repeat, not, technically Empress Regnant of the HRE, she's the Empresss Consort once FS has become Emperor. What she is undisputed Queen Regnant of is Hungary and Bohemia, where Salic Law does not apply.) (Hence when we speculate how Catherine the Great would have fared if she hadn't married (P)Russian Pete but had remained Sophie von Anhalt Zerbst and married Heinrich instead, there's no scenario where she could have done what she did in rl and end up as single Queen Regnant. (Regent, yes. Queen Consort, yes. Queen Regnant, no.)
I think while going for hypothetical daughter's children would have been possible if there were no alternative male heirs, the fact that Fritz not only had three living brothers but that two of them (AW and Ferdinand) had themselves sons, not to mention that his Great Grandfather, the Great Elector, had spawned a whole separate line of Hohenzollern in his last marriage (the Schwedt cousins, one of whom Bach wrote the Brandenburg Concertos for) would have meant such a scenario need not apply. Besides, and speaking of the Schwedt cousins, another of those married in RL Fritz' sister Sophie. Presumably he could have married a daughter to a Schwedt cousin instead in that alternate timeline in order to reunite the two Hohenzollern branches and avoid any foreign prince getting to be King in Prussia.
(Not 100% sure he'd have done it if only because he didn't have a very high opinion of the Schwedt cousins, but it would have made 18th century dynastic sense.)
Me:
I always get confused about where Salic law does/does not apply, and went for 'doesn't' here because of Anna of Prussia--and the assumption that the HRE was an elective oddity.
Well, probably for the best that Fritz can't try and create a perfect grandson via his daughter, as the daughter tries to take MT as a role model. And I think she'd be the wrong age to marry Paul of Russia, however much he might like the idea.
mildred_of_midgard:I'm reminded that in the 13th century, a Hohenzollern Swabian Duke got permission from Rudolf of Habsburg that a woman could inherit if the male line died out...but I agree that nobody would see having 3 brothers and however many nephews as the male line dying out
Back at the original
selenak comment...mildred_of_midgard: Ooh, this is bad. Very bad. :P Poor kid, whoever he/she was.
I agree with everything you say here, Selena.
the kind of extrovert adorable imp that apparently both AW and (at a guess) Henricus Minor were
Yeah, and personality aside, I'm betting Henricus Minor benefitted from not having all the weight of expectations of being the heir plus being seen as a potential threat.
I agree with your guess that he was an extravert adorable imp, because Fritz's Very Bad Condolence Letter to Ulrike on HM's death said that HM was a lot like his father, minus his father's shortcomings (omg, Fritz).
MT: I pity any offspring this man might have. However, I suddenly see some potential for a return of Silesia a generation later if the kid is properly influenced. Maybe more. Hm. Which of my daughters might make a good Queen in Prussia?
Lololol! Tu felix Austria nube!
So which of the daughters *would* she choose as queen in Prussia?
Or, conversely, who does Fritz's son marry, if not one of MT's daughters?
cahnMy first kneejerk response was "OMG that poor kid!!" :/
Also, if he has a daughter, he won‘t feel threatened by her or want her to be exactly like him or would resent her if she is like him. If this daughter has some musical talents and likes literature, he might be tempted to make her into the second Wilhelmine, and if she doesn‘t and is more like EC, he still won‘t be that disappointed because of the general low opinion of women.
Oh man, I never even thought of the possibility a kid of Fritz's could not have a terrible time, but... this sounds plausible. Ironic that it's because of misogyny!
But if the kid is more an introvert, let alone shy: DOOOOOOM. TRAUMA.
:(((((((((((((
I also imagine in my head the boy being more like FW in tastes and wanting, like, less French and more cannons, and Fritz being very awful to him in the FW-reverse way, because how DARE this kid not appreciate all the things that he would have loved to have!
AW: If Fritz has a son, does that mean I can divorce Luise and marry the love of my life? (Whoever she might be, either Sophie or Mina.)
Fritz: NO.
AW, Heinrich had to marry and it sure wasn't for the succession!
Me:
I return, properly logged in (I didn't take my laptop to Berlin), with the urge to give these unfortunate hypotheticals names. My instinct is to go for a daughter b. 1737 and a son b. 1739 (after, I think, FW1 has sent them a velvet bed?) and I fear that there is no chance this unfortunate son doesn't get forced to be another FW, as the other name option from FW1's ancestry is George. The daughter is probably another Sophia, and I feel like Frederick would fight for a Wilhelmine.
selenakNow that's slander. ;) Going by the names FW gave his own kids, including the son he had before Fritz (and even before Wilhelmine) died as babies, he was entirely willing to combine "Friedrich" with "Ludwig" instead. So a grandson could have been called "Friedrich Ludwig" as well. FW naming the younger sons after his pal and competition from Saxony (the August in both August Wilhelm and August Ferdinand) would indicate a readiness to honor a living neigbour as well, and given this hypothetical kid would be half Braunschweig, he might go for "Friedrich Karl" or "Friedrich Ferdinand" (both Brunswick names).
(If Fritz gets to choose at all, I'm tempted to go with "Karl Emil", which is the name he suggested for the baby his sister-in-law Luise was pregnant with when AW died. Back in the day we thought that was random, but since then we found out Karl Emil was in fact the name of the Great Elector's original oldest son, big brother to future F1, who died young, and thus already a Hohenzollern name.)
The daughter is probably another Sophia, and I feel like Frederick would fight for a Wilhelmine.
Agreed on both counts.
If there's a son born in 1739 specifically, btw, it might cause some interesting developments. Because that year the father/son relationship between FW and Fritz took another sudden downturn in tandem with FW's health, so much so that according to the gossipy envoys and according to the Fritz and Wilhelmine letters, FW even made noises about the succession. Now he couldn't have altered the succession from Fritz to AW, no matter what he threatened, without Imperial backup and the Diet of Regensburg agreeing. But if he suddenly has his first grandson in the direct line of descent, not by a daughter, then either he's so pleased that Fritz is spared a few paternal rants, OR he makes noises about "what if I alter the succession so your son inherits directly from me, provided I stay alive for the next two decades as well?", which would be a bit easier to do, I suspect (though I guess would still need Imperial backup.)
Me: I apologise to FW most heartily! I also note that we are entering the period of three given names, so possibly Elisabeth Christine manages to get a Sophia Wilhelmine Elisabeth, or Sophia Wilhelmine Antoinette, in there. And this, of course, adds additional options for little baby Friedrich Karl Emil.
Given FW's...FW, possibly he starts off by sparing the rants, then goes to "what if I create him my heir? What THEN?" and then dies before he can change the succession, and submit Prussia to the rule of an infant who cannot yet consume solid food. (For added irony, possibly FKE either dies before Fritz, or survives him only a little, so rule by infant or toddler is only put off for a generation.)
So, here they are, Sophia Wilhelmine Antoinette of Prussia, b. 1737, and her little brother Friedrich Karl Emil, b. 1739, ready for the horrors of the 18th century. Specifically...marriage.
I wanna marry Friedrich Karl Emil
This is an aggregation (and amendment) of several of my comments about who would marry hypothetical-FKE. This assumes that FKE is born in 1739, and marries in 1765, aged 26.
Catholics
Holy Roman Empire! The clear option here is Maria Elisabeth of Austria, b. 1743. Historically, she never married, and was Abbess of the Theresian Institute of Noble Ladies, but she was certainly considered and negotiated for, they just never found anyone they liked, and then she caught smallpox, which scarred her for life. According to Wikipedia "In adolescence, Elisabeth was described as unstable and eccentric, without any particular interests. She was fun-loving and neglected her studies. She earned her mother's censure by liking gossip and mocking everyone, offending her siblings and attendants with ironic comments. Her staff changed often, as few could stand her. Elisabeth was a difficult child to raise, and five ayas were appointed in turn to educate her; even her confessor needed to be changed once. Her mother considered her childish and immature, calling her eine Kokette der Schönheit ('a pretty coquette'), observing: 'It mattered not if the look of admiration came from a prince or a Swiss guard, Elisabeth was satisfied.'" Would this put Fritz off, or not more than 'having MT for a mother and being Catholic'?
Poland-Saxony. Maria Kunigunda of Saxony b. 1740! Historically failed to marry after she was too shy for Joseph II, and everyone assumed she had something wrong with her. Lucky escape, says I. Instead, she became an Abbess, not for vocation, but for lack of other options. She was the youngest of sixteen, and while two of her elder sisters remained unmarried, the others became Dauphine, Queen of Bavaria and Queen of Spain. While religion remains an obstacle, her father converted to get Poland, so converting back might be easier. As an Abbess, she was apparently an excellent businesswoman, and also benefited from her parents' interest in their children's education.
Bavaria. Maria Josepha of Bavaria, b. 1739. Could it be worse than marrying Joseph II? It would also help Fritz in the War of the Bavarian succession, which doesn't happen until the 1780s, but is probably visibly in the offing.
Protestants
United Kingdom. There are a couple of options here, but I don't think Fritz would or should go for them. In the early 1760s we have three unmarried daughters, Augusta (b. 1737), Louisa (b. 1749) and Caroline Matilda (b. 1751.) Louisa has tuberculosis, and her marriage negotiations came to nothing because of her health issues. Caroline Matilda is 13, and might be a possibility. Augusta, meanwhile, would be a better possibility, but historically, of her four sons, three had significant mental and physical health issues, with two of them being excluded from the line of succession, and it seems unkind to put that possibility into the Prussian court. She also greatly disliked anywhere but England, and took out her bad marriage on her daughter Caroline. Obviously, none of this can be known at the time, but still...
Mecklenberg-Strelitz. Christiane, b. 1735, received a comprehensive education, but never married. Elder sister of Queen Charlotte of the United Kingdom, which might be a benefit. On the older side.
Brandenburg-Bayreuth. Elisabeth Friederike Sophie, b. 1732. Historically married in 1748, so is not really still available by the time her little cousin reaches adulthood, but I could see Fritz arranging the match much earlier because Wilhelmine's daughter.
Brunswick-Wolfenbuttel. The daughters of Fritz's sister! Sophie Caroline Marie, b. 1737. Anna Amalia b. 1739. Elisabeth Christine, b. 1746. Very little to choose between them, and of course Elisabeth Christine did marry FW2, but with consequences I doubt could be averted with a shyer, more introverted husband...
Brandenburg-Schwedt. Daughters of another sister! Friederike b. 1736, Anna Elisabeth Louise b. 1738 and Philippine b. 1745. Historically, Anna married her uncle Ferdinand in 1755 and they had seven children.
Denmark. Sophia Magdalena of Denmark b. 1746, but she was betrothed to Gustav III of Sweden aged 5 by the Riksdag of the Estates. Her younger sisters Wilhelmina b. 1747 and Louise b. 1750 have nothing against them, and would get Denmark that Prussian alliance that Sweden has...
Sweden itself hasn't got a princess until 1753.
What are people's thoughts? Who, and what important notes, have I missed? Am I too visibly biased in favour of Maria Kunigunda, except for her name?
Whether SWA marries, I could not say. I feel like either Fritz finds her the most brilliant match possible, or her makes her a lady Abbess so that she can stay with him like Wilhelmine couldn't, and she gets to have an affair with her SIL.
(I also like the idea that, whatever FKE's personality was, he has the reproductive luck of the father of the current King of Sweden, who had four daughters before they managed a son, and died nine months later)
Heinrich, in 1786, made regent for a small child: My time has come.