this is a bit hyper specific, but the one thing I'd want more than anything else in a KH3 re-release is a significant tweak to boss behavior. Rock Troll, Tornado Titan, Angelic Amber, King of Toys, Chaos Carriage, Grim Guardianess, Marshmallow, Skoll, Catastrochorus, and even Xehanort himself occasionally go into a neutral, inactive state in the middle of combat, where they walk/float around waiting for you to attack. This actually makes the game harder AND less engaging. I don't know how many times I've been standing there like an asshole holding up my Countershield waiting for the Rock Troll to fucking attack me so I can build up my counter, only for him to start pacing left and right... Are we fighting or not?!
Bosses should maintain a rhythm of aggression that the player can learn and engage with. I absolutely believe such a tweak could be made even far after release.