I'd love to see KH3 reintroduce the way KH1 gated certain ability upgrades (specifically, spell upgrades) behind sidequests. KH2 attached some bonus level upgrades to optional bosses, and I'm into that; also, obviously each keyblade contains a passive ability and that includes the optional ones.
I kinda hope KH3's equipment system gets simplified and made more similar to the excellent one from Re:Coded, though. I don't think accessories or armors add a lot to the game (nor, to be honest, does having to budget your passive abilities with AP), but I love how Re:Coded took KH2's concept that every keyblade should have some unique advantages and ran with it so each one almost feels like equipping a different job. That (as well as differences in length/crit/interrupt and stuff) is way more interesting than different levels of strength/magic in the keyblade, IMO. Leave the stat boosts to leveling, please.