Got a minute? This might take a while.
1. Vanitas/Maleficernt/Pete trio game
Vanitas joins forces with Maleficent & Pete, becoming a dark mirror to the Sora/Donald/Goofy party. Other Disney villains act as guest party members. New game mechanics include minion summoning & "Boss Forms", meant to represent how RPG villains change form mid-fight.
Minions:
Vanitas=Unverse
Maleficent=Goons
Pete=Beagle Boys/Weasel soldiers (from Prince & the Pauper)
Jafar=Mamluks (bad guys from tv series)
Hook=Pirates
Hades=Undead centurions (I'm aware those are roman, not greek...)
Xanatos=Steel Clan
Shan Yu=Huns
Kuja=Black Mages
Boss Forms:
Vanitas=Chimera (fusion of many unverse body parts into 1 monster)
Maleficent=Dragon
Pete=Julius (comes with a retcon, it already seems semi-canon that Pete is his last name anyway)
Jafar=Snake/Genie
Hook=flight (Tink's been put in that lamp 3 times, there's gotta be enough loose dust in there for him to use)
Hades=Red Flames
Xanatos=Battle Armor
Shan Yu=Horseback
Kuja=Trance
While Vanitas' storyline involves the darknesses, Maleficent's is about her & Pete losing control of the heartless (hence, their hiring other henchmen) to some external power, which is eventually revealed to be the Horned King taking control of the heartless with the Black Cauldron's magic. As such the heartless are serving as enemies in this premise but there's also local forces that must be contended with as well:
Radiant Garden=SeeDs
Agrabah=Palace Guards
Neverland=Lost Boys
Olympus=Spartans
Castle Wyvern=Wyvern Clan (more survivors than in canon)
Land of Dragons=Chinese Army
Prydain=Horned King's Men
Disney Castle (conclusion to Disney villain plot)=Musketeers, Broomsticks, & Gummi drones
2. Kingdom Hearts Warriors/Musou
This has been on my mind since Hyrule Warriors & further deepen by Persona 5 Strikers. KH has already had multiple instances of musou like battles (1,000 heartless war, the space invaders in BBS) that doing a full game like this doesn't seem that much of a stretch. I see a few possible premises they could go with this:
A. Set it during Days & have the Org be the playable characters with nobodies as their allied army. After all, we don't see every single 1 of those 358 days, only about 80-90, so they could make a whole 'nother game out of the skipped days.
B. Focus on the Guardians of Light with dream eaters as the allied army. Lord knows this series is gotta have issues juggling so many characters going forward so games like this are a great way of making sure everyone's putting in the hours.
Ideally there would also be playable Disney characters in this, but that depends on how willing Disney is on playing ball. Speaking of which.......
3. Dissidia Kingdom Hearts
You know what, I don't wanna give up on this idea, especially not after how NT ruined DFF. Way I see it, the worst case scenario is the playable roster is all series original (really the 7 vs. 13 thing is such a Dissidia set-up anyway) with Disney characters acting as summons. Best case scenario is that "Dissidia Kingdom Hearts" is basically code for "Disney Smash Bros." & features loads of playable Disney characters. The Sorcerer's Arena mobile game allows for all manner of hot Disney-on-Disney action so why can't KH? Oh, & speaking of Smash Bros.....
4. Nintendo Hearts
F*** it, why not? A Switch exclusive KH game featuring Nintendo worlds in place of the usual Disney affair. We got Grand central Station in the series now, use that as a hub connecting assorted Nintendo games together. Maybe Litwak got a bunch of those old store demo console units or something.
That said, while there aren't Disney worlds, Disney characters still show up in the Nintendo worlds because.......reasons. Maybe it's just data simulations uploaded from Jiminy's journal I guess. Point is each world has both a Nintendo & Disney party member for maximum crossovering:
Mushroom Kingdom=Mario & Mickey
Great Sea=Link/Tetra & Peter Pan
Zebes=Samus Aran & Buzz Lightyear (as in the Buzz from the in-universe tv show, not the toy)
Kanto=Pikachu & Stitch
Ylisse=Lucina & Mulan
Lylat=Fox & Baloo (Tailspin version)
Skyworld=Pit & Hercules
& Sora would have his own counterpart in Captain N, which is a deep cut but he fits the mold of an OC hero going to different worlds based on established properties.
5. Disney Infinity Complete Gold Edition with bonus KH content
So there's a studio called Play Magic who are working on a "Disney 3D action game" remake. More likely it's an HD remaster of Epic Mickey 1 but I would love if the Gold editions of DI on Steam (which don't require figures) got ported to current systems plus a bit of KH-related content to sweeten the deal. Doesn't have to be much, a playable Sora, 1 or 2 heartless enemies & Traverse Town environment for the toybox mode would suffice. Maybe a spawnable boss like Ansem & a couple of tracks.
6. Game featuring Disney TV worlds
I've hinted at it a couple of times already but KH is really overdue tapping into the well of Disney's television history. Ducktales, Gargoyles, Gravity Falls, Kim Possible, Tailspin, Darkwing Duck, Adventure of Gummi Bears, Chip & Dale Rescue Rangers, & the tv versions of some of their films are all ripe for the picking.
7. Metroidvania in Castle Oblivion
An idea that occurred to me while playing Bloodstained, CoM really could've been done as a sidescrolling Metroidvania instead. Maybe this time around it's about Kairi's training under Aqua since Aqua's in control of CO now. Like in any good metroidvania, Kairi has to aquire new skills/abilities/items to move about the castle. For example in order to explore the underwater Atlantica wing, she needs a mermaid transformation which is found in the Mermaid Lagoon of the Neverland wing but traversing Neverland requires flight which is achieved with the flying carpet in the Agrabah wing. Stuff like that.
To add some stakes to the plot, Lauriam & Elrena try to take over the castle again & gets Ven involved.