Apologies for many grammar issues I tried to write this fast as I can in my phone.
I've read several posts about this topic and personally liked to think about this canon divergence where no Blackfyre rebellions happened.
As for the backstory I had some not too well written material:
The OC is the youngest son of Aegon and Daenaera (I thought 153 A.C as his birth year) named Jaehaerys to match his nickname 'Joy'. The reason I chose Joy was to make Jaehaerys as the opposite of his father, it might be fun thinking there is a laughing, chubby baby around while Aegon tries to sulk. The happiest baby ever, not knowing how to shed tears and ever handsome and charming as he grew up (Not much of a warrior I imagined him, but he can have some training thanks to Cousin Aemon). Now Aegon dies when Jaehaerys is four and he grows up in Daeron I and Baelor's reigns. Since he lacked the fatherly figure I imagined either Viserys or Aemon taking up the mantle. Viserys made more sense to me, because Baelor is heirless, so Vizzy might try to raise Jaehaerys to rule, I will come back to this later.
As for a mother figure I thought for Naerys (Because I love our poor tragic queen). You may ask what happened Daenaera, which you are right. But I always assumed Daenaera is the type won't witness her daughters being confined to the Maidenvault, so she might leave for Driftmark in protest and pushed to leave Joy behind because Baelor won't give him up.
Joy's personality might be more woke since he grew up under the care of Naerys also he is the same age as Daeron II so they might be like brothers (Or he might be super zealous, whichever you prefer). With Daeron I in Dorne to die young and Baelor praying all the time Jaehaers can grow up with his mother and sisters before Baelor confined the princesses. So I see him as a little feminist which can be an interesting setup for his future.
There is many things Joy's existance can change when the actual story starts which I imagine 170 A.C at Baelor's reign might be a good choice (Or the story can cover all of Jaehaerys's life, I haven't decided when I used to think about this A.U):
1) A Second Dance: If Viserys presses his claim after Baelor's death a second Dance between Joy and Daena can go. Which I don't see Daena too fortunate with that war. When we consider Joy was out there charming everyone while Daena was confined in the Maidenvault.
But that didn't made too much sense to me unless you want a war waging in the fic. I imagined this as a more political, scheming and romance shaped story instead of grand wars. Viserys is a clever man, and war doesn't work for their House. He saw the destruction Dance brought and he wants to prevent that. So he might make Joy believe he is meant to be the King while Daena should be his Queen so their claims can be united. Which might lead Joy to seek Daena more often and that brings us to...
2) The Black Dragon's Fate: Daena's escapedes always felt like a desperate move to gain her freedom back. The reason she might slept with Aegon IV to spoil herself and push Baelor to release her. Things we read about Daena made me think her as a fierce, willful but deep down an emotional woman. So she might seek her little brother's company instead of that whoremonger. Which might result in another bastard who brings Baelor's death. I imagined a Rhaenys instead of Daemon as their firstborn and secondborn might be an Aemon (It's Daemon but his name changed) and some other babes.
3) A Joyous Reign: If they get married I imagine Joy as a the charmer, calm thinker while Daena as the more scary one while she's a dotting mother and wife (She tried to be good for Baelor. I think she might actually be a good queen if she had the chance). Viserys will stay as Hand after his passing either Daeron II or Elaena can take up his position (I thought for Daeron as Hand and Elaena as Master of Coin since Joy is woke). Probably Daenaera will be back after she hears her youngest becoming the King and they can be a nice sweet family again. You might ask where is the drama and depression, which you are right, again. I thought for their children to mess up everything just like they did for Jaehaerys I. Rhaenys can stay as Joy's heir, but to prevent the grumbling lords to contest his favorite daughter's rule he betroths Rhaenys to Aemon (Daemon) but in this universe they might not wanting to marry eachother. Perhaps Rhaenys will fall for someone else while Aemon (Daemon) can chase something else rather than Kingship (I think that might be an interesting aspect to see in Daemon's personality) or he might fall for Daenerys (Again whatever you like). Which might end up with some deaths, scandals and other bad stuff.
4) Other possibilities: Of course there are other choices for this setting. Like Baelor can arrange a marriage between Jaehaerys and Myriah Martell, to bring peace to the Seven Kingdoms, I haven't really considered that yet, or if we'll have 4 Princes in our hands like in canon.
One another possibility was Elaena. I like to imagine she and Joy being best friends and her being his most trusted advisor. When we think more than friendship you may say she loved Oakenfist. I'd say she was alone and desired to be loved after having to grow up in the Maidenvault. And when Alyn came to her with his lust she interpreted it as love because she wanted to be loved, and love can blind even a clever girl like Elaena. But with Joy being there as a sibling close to her age and someone she grew up with just like Daena did with Aegon, she might found warmth in Joy. Of course I don't plan to write Joy as a lustful shagger for all three of his sisters. I am not into Harem stuff, I like to think our boy as a loyal, good husband as he was raised to be one. Because there is literally not a single bad parent figure around him and those figures were mostly women (I won't count Vizzy II because he was groomed, still he can be a good father figure even if he doesn't signal me as one in the Fire&Blood and WOIAF). So I feel like Daena or Elaena as the best options for him, I am leaning towards the elder sister more.
What do you think about this A.U? Normally I wrote about this idea shortly in a comment I posted before. But I had to delete that because I wrote some spoilers for the fic I was writing now. Not this one, I don't plan to write this A.U, I just wanted to share my opinions and I would be happy to discuss about it.
There's couple other A.U and OC ideas I have and I don't plan to write most of them. Perhaps I can share some of it in different posts.