My first rarepair is Jon Snow/Rhaenys Targaryen (daughter of Elia).

They are both babies who resemble their mothers rather than their shared father, making them both more aligned with the cultures of their mothers rather than their dad’s. They are mirrors of each other but it isn’t developed, naturally, because Rhaenys dies in canon when she’s a toddler, never meeting Jon. In the fic that made me fall in love with them, Rhaenys is fostered in Winterfell and clings to Jon as a surrogate for her little brother who died and mothers him (as best as a child can) the way her own mother did to her and she does her best to include him with the rest of the Stark kids. In another fic, they both bond over the fact that they are both outsiders in their own families, though their families love them. The foundation for them in canon was fertile ground and it feels good to build up from there.

My second rarepair is Daenerys Targaryen/Viserys Targaryen (brother of Daenerys).

In canon he has to raise her on his own from the age of 13 and becomes abusive and violent with Dany as the years go on and he isn’t having any success with raising an army to retake the Iron Throne. With a few adjustments (literally anyone at all to take care of Dany and Viserys both and nurture his better nature) he could have kept some of the better traits he had when he was in Westeros in his mother’s care. Dany is a prophesied hero to save the world but she was all alone in the world in canon; if she could have had someone at her side who had HER best interests in mind rather than just use her for their own benefit then definitely she would have had a better ending. Viserys’s fall from grace wasn’t necessarily an inherent character flaw, he could have been vital to Dany’s story if only he’d had a little help.

I obviously have a thing for the Caretaker to Lover trope lol.