I really want a female villain, tbh (who is probably the exact opposite of Xehanort in every way). I know there's a good chance Square Enix could mess this up (as they always use tired stereotypes when it comes to a lot of their women characters), but if they did it right it could be really something special.
No more clones of people...
No more Keyblades being handed out like candy...
I'd probably want a smaller series, with no spin-offs, that's mostly all planned out from the get-go and easy to understand the order of games. For example Kingdom Hearts: The Ties That Bind 1, Kingdom Hearts: The Ties That Bind 2, Kingdom Hearts: The Ties That Bind 3... you guys get the idea.
I want a complex story, but not a convoluted one. So that means no twists just to be "shocking", that only have a short moment of satisfaction... before you realize just how much they changed/damaged things.
And no more Nomura writing the story backwards, either.
No more man behind the man...
No more console hopping.
No more data stuff (this is a personal preference of mine, and I understand if people don't agree with it; I just hated the inclusion of that into this franchise).
Likewise, as much as I want certain things to get explored more (*cough* intermediary weapons people use before getting a Keyblade *cough*), I think that would defeat the entire point of a clean slate, so things like Nobodies should probably go, too.
As for things I want:
Character, character, character. Not so much focusing on the plot and its mechanics this time, but giving me characters actually interacting and proving that they'r friends and know each other.
And in that vein, I also want diverse characters who are really different from each other, and a TON more girls.
More human struggles that we can relate to, not "the darkness was ripped out of my heart and became its own physical entity and now everyone wants me to fight him" stuff.
Kairi. Since this first saga treated her so badly, I think it would only be fair if the next one actually handled her well.
Disney to have a super important and plot relevant role again, and for the games to feel like the Square Enix/Disney crossover that they started out as. Because more and more, it's been tipping towards the Square side and forgetting the Disney completely. In fact, I might even want the next Big Bad to BE originally from something Disney (and actually stay the main threat, unlike with Maleficent).
I think dealing with familial ties for once could be great, and a nice change of pace. Friendship should always be the main theme of KH, sure, but we could still explore family stuff some--even if it's just through Ansem the Wise and Ienzo or Master Eraqus and his pupils.
Tbh, I don't even know if I want Kingdom Hearts itself to have anything to do with the next saga. It's so over-done at this point... So something new and exciting that everyone's fighting about would be great.
As for the plot, I think I'm team "Someone trying to destroy Keyblade wielders, in thinking they've brought too much destruction to the world." But I wouldn't mind a Light villain, but that would have to be handled EXTREMELY well--otherwise it could destroy everything KH has done beforehand.
Depending on how KHIII ends... Maybe the world being destroyed, and us dealing with that--living in a world devoid of light like that--and trying to fix it?
Or the opposite of that: All the worlds being connected again, making the next KH games all open world, with the potential for some real crossover stuff with the Disney properties (like all the movies that took place in Paris being grouped together, and whatnot)?
I also wouldn't mind seeing Sora go dark for a hot second, but that's probably never going to happen.
But speaking of Sora: I just want the next saga to let him be smarter, have character arcs, and let have true emotions about things and trauma again, instead of just making him happy 100% of the time.
Oh! This is an idea I think everyone's had and is on board with: Our heroes finding another part of the multiverse that they never would've guessed at before, and getting to go to all kinds of new and crazy worlds because of that. *Cough* Finally give me Final Fantasy worlds that way. *Cough*
Branden from KHUnion's idea could be interesting, too: He proposed the idea that what if what we're fighting next time isn't sentient at all--and doesn't have a mind or a heart, but just lives to eat worlds to survive, and thus Sora can't reason with it at all. It would probably be something akin to Sin from FFX.
Edit: Though this really wouldn't work with KH's vibe at all, and I know that, I once thought some Faerie lore stuff could be cool for the next saga (maybe even with them being the next bad guys?), since a lot of their stories have to do with doors--that KH already does a lot with, because of the Keyblade and stuff. But meh.
Edit 2: It would also be nice if we could go to the same Disney worlds less--especially if we're going to relive the same plot from it over and over again--but it's not a deal-breaker for me if that ends up happening.
And next time, if at all possible, I'd love it if I could take my Disney party members with me places...