I want KH3 ReMind's photo mode to return and I want 0.2's customizations to return. It was so fun to mind of dress Aqua up, and I think it. would be pretty cool if they exchanged the puzzle crowns and the hidden Mickeys with costume pieces. Just let us find them all and customize our characters in any way we want.
I'd also welcome a fighting mode of some sort, or a mission mode like in Days, where you simply select a character and go. Thinking about it, I think it would be fun to be able to play as any character you want once you complete the game.
Plotwise, I would like Aqua and Kairi to have a bigger role, that's just my bias speaking. I also hope that we get to play as multiple characters again - and not just in battle.
I also want them to modernize the games even more in terms of the system and settings. Let me set the volumes of the effects, music, and voices however I want.