Ignoring the obvious ones that objectively improve their games (netcode, crossplay, good tutorials, etc.)
I'd like to see more games with assists try out the custom assist feature from Skullgirls. It was a really cool idea (even if it's a pain to balance) and it lead to players being able to really express themselves with their teams. Also on a similar note, Skullgirls' variable teamsize/ratio implementation. Both because it gives players even more control over how they make their teams and from what I've seen it's a decent way to sell a game to someone that's brand new to team fighters ("no you don't need to learn 3 characters, you can just learn 1 and have a good time.")
I guess these can't be universal because I don't want every fighter have teams, but it'd be really boring to have another post about good rollback netcode and crossplay.