I made a massive effortful post about my KH3 wishlist and I'll be damned if it doesn't get replied to. so I'm reposting it here
Overclock system from Re:Coded - essentially, each Keyblade levels up and unlocks new passive abilities for itself, and each one is the rough equivalent of a Job from the FF games - it makes equipment really matter, and it did a better job than KH2 of making it so that you couldn't just get by on equipping your strongest keyblade.
No other equipment/no equipment synthesis - well-differentiated Keyblades would be enough, to me. The equipment system in most FF games has been micromanagement for micromanagement's sake and hasn't made enough of a significant difference (outside of building up lots of elemental resistance for specific boss battles on harder difficulties) to be worth keeping around. And item synthesis in KH1/2 was never actually that fun.
Command Styles from BBS - these were a great way to add immediate dynamism and variety to combat.
Melee combat most akin to KH2 - lots of juggling and hitstun and dodging/blocking/countering, plus a strong variety of equippable combo starters/finishers. The basic melee combat in BBS/DDD was pretty dull even though the the Command Deck is boss.
BBS passive-ability system - keep it simple, stupid. None of that AP shit, please (or DDD's weird way of handling it, kthx).
Actual magical spell progression - I fucking love the Command Deck and ability melding from BBS, but when you can fairly easily grind your way up to Firaga in the first hour of the game there's something amiss. KH2's spell combo system was pretty danged great, too.
BBS/DDD level of ability variety - yes, I know this kind of contradicts the point immediately prior to it, but perhaps the Command Style system could be a way to have Sora's Firaga temporarily turn into Megaflare (or similar things). Forcing you to combo your way up to the best attacks in the game would be a good way to add balance *and* skill depth; the fact that in endgame CoM/BBS/DDD you can basically start every single fight off with one of the most powerful abilities in the game and just keep spamming power moves until everything is dead [yes, caveats go here for Critical difficulty] shows that some more finesse is required. But those moves absolutely ought to exist in the game, and it ought to be easier and easier to get to them the more powerful you get - moves that occur infrequently and feel basically like limit breaks in the early game ought to appear really easily in the endgame, but you should still need to combo your way up to them.
Multiple, well-differentiated playable characters - Re:CoM, BBS, and DDD have all had this. It's time for KH3 to have it as well. Multiple campaigns or no, there should be a massive simultaneous party and you should get to play with a large number of potential protagonists, IMO. Different characters should platform/dodge/block differently, should have different melee combat, and should have different special moves, all while adhering to the same underlying fundamentals.
Arena world with a shitload of optional bosses and battles - KH1's Olympus Coliseum, KH2's Garden of Assemblage, BBS's Mirage Arena. This is an absolute goddamn necessity. Ideally it'd be possible to re-fight any boss in the game. Optional bosses should be challenging as usual, but shouldn't have bullshit command-locking attacks because those are dumb and immersion-breaking as hell.
Multiplayer versus mode a la Dissidia - KH2's single player combat actually has a lot of the necessary ingredients for such a thing (lots of enemies have Sora-esque combo starters/finishers, air recovery, juggling, and so on). Let us play as tons of heroes and villains. It'd be a blast. BBS/DDD both had networking features; it'd be backsliding for KH3 not to have some form of multiplayer.
Final world of complexity approaching or surpassing KH1's Hollow Bastion - KH1 Hollow Bastion is the only seriously complex world in the entire series. It's massive *and* dense, full of little side puzzles, constantly doubling back in on itself, and feels genuinely like a dungeon.
Star Wars, Marvel, and Pixar - because this is Kingdom Hearts and it's supposed to be totally crazy. Just give us Asgard, Dagobah, and maybe Syndrome's island from The Incredibles, please. Fantastical environments are key, I feel (Cloud City or basically any non-Asgard Marvel-movie stuff would be dull).
Bigger party size - I shouldn't have to swap out Donald or Goofy when I visit a Disney world. Just add on the guest character - you have the RAM! And the endgame should give me a party of all seven light wielders, dammit. Being able to swap control between a bunch of different characters would be fucking great.
Wrap up the Xehanort stuff - it's been fun (and ridiculous), but I really don't want this level of convoluted storytelling from here on out. KH3 should establish going forward that there's a full and diverse cast of KH-original characters (and a full set of KH-original worlds) existing in this universe that can be involved in future plotlines, but this should mark the end of the everybody-is-Sora-and/or-Xehanort shit. The work of populating and fleshing out this universe is done (whereas early in the series it had to be done with lots of guest-starring FF characters) - things following KH3 should take that as read and tell stories with it.
Massive amounts of endgame/postgame content - Give me reason to revisit and explore old worlds (like chests that were previously unreachable). Give me lots of sidequests and superbosses (and make them all genuinely rewarding - you should get material rewards that make Sora more flexible/powerful, not just Journal entries).
New Game Plus - because it's the 21st century. Figure out a way to retain some of Sora's abilities and equipment while resetting his level back to 1.
No more Gummi ship - self-explanatory. Just use the keyblade glider stuff. Sora's overdue for some armor anyway.