I'm gonna start with general stuff and then go into detail for each fandom.
Things I don't like/find squicky:
Also foreign languages
Moral shades of gray
Fandom specific notes:
Kel! PotS was one of the first series I totally fell in love with and it was totally because of the friendships. I adore Kel and Raoul's father/daughter-esque relationship. I love the way he treats her as a equal, teases her and protects her. He's a great mentor and I would literally just read pages of them interacting. I'd also love to see something about how they worry over each other, after all it's a dangerous world out there and they're both pretty much on the frontlines. As for Neal, I'd love something about them being protective over each other or having each other's backs, especially in unexpected ways. Like I adored that scene in Lady Knight when Kel went on a walk with Neal because she knew he was going to lose his temper and needed space to cool down.
Rivers of London:
Sahra Guleed is just so cool. I would love to see the evolution of her friendship with Peter, her getting involved with 'weird' cases and the ways that she copes with knowing about magic.
I live for Ronan & Nita interaction. They're just so snarky and I think they have a lot in common in terms of both dealing with a lot of anger. Plus there's that time when Nita all but brings him back from the dead. So Nita and Ronan getting in trouble together, raising hell, getting into heated debates, all of the above? As for the Nita/Ronan/Kit, again I think it's an incredibly interesting dynamic. I'd love to see something about how they balance each other out, how they navigate their relationship and/or how they rely on each other.
Billy Kaplan is just trying so hard. I'd love to see, again, how he relies on the people around him. He's this incredibly powerful superhero but at the same time, he needs the support of his friends, his brother, his boyfriend. I'd especially like to see him coping with something that he can't fix with magic, see how he copes with the powerlessness, knowing how powerful he is. Also domestic fluff is literally always an option, especially if includes multidimensional diner tours.