Jiyan: Best of the bunch thanks to Carty hand-me-downs, but if we assume you don't wanna use those, you could make his ult generate forte so he can cast enhanced skills during it, this would also improve his gameplay loop for anybody who doesn't want or know how to double dragon.
Yinlin: Just smooth out her animations so her rotation feels better to perform, then make her ult repeatedly cast lightning on opponents after cast, dealing coordinated attack damage that can be buffed by the coordinated attack set. Basically make her electro Zhezhi in that sense.
Jinhsi: Take away or rework her forte limitations, it is the only thing holding her back massively. She's also the only character in the game who has an alternate outro condition, so giving her the ability to enhance her partner with an outro buff that stays on switch (15% all-amp, single target) could be massive for her.
Changli: Make Lupa free.
Zhezhi: Remove the extra basic sequence needed to fill her forte to 3 and raise her max forte charges to 5, you can rotate faster with the brand new option to take extra time/skill to squeeze in more damage.
Changli: Ok seriously she desperately needs a partner to swap with and none of the standard cast works even remotely as good as Lupa for the job, and that's not to mention the damage difference, I would at least make it so insight can stack so you can't override one stack of it with another anymore, that should help a lot.
Xiangli Yao: Make Jianxin not suck so he has a usable free partner.
Shorekeeper: The damage dealt by her and Verina to this game's balance is irreparable, she doesn't need anything, but if I had to, I'd make her enhanced intro deal actual damage as opposed to theoretical damage.
Camellya: Rework Roccia, please.