My ideal Kingdom Hearts IV, changing it up a bit.
Firstly, I'd do a personal story spin-off game starring Lea on his own personal adventure traveling throughout the Disney channel tv show worlds.
Then, onto KH4.
Main villain: Oswald the Lucky Rabbit (Personal vendetta against Mickey), w/ seeds of the next big OG KH villain throughout.
Party Members: Max Goof (trying to save his father who's been captured with Mickey, Donald, etc.), Darkwing Duck (A lone vigilante wanting vengeance against Oswald, who destroyed his world/family.)
TEN DISNEY WORLDS
Moana - The Sea (I'd make this the first world. I would love KH3 to end with the trio, or whoever lives, on their raft finally, and then running into Moana on the seas, leading directly into KH4 before all of the madness of the main villain's plot ensues.)
Atlantis - Underwater Kingdom
Bug’s Life - Ant Hill/Bug City made of cardboard and other items (Wayne Szalinski from Honey I Shrunk the Kids uses his shrink ray to accidentally send the boys to the world of a Bug's Life)
Treasure Planet - Space
Coco - Land of the dead
Zootopia - The city with versatile animal environments
Inside out - Kairi’s Mind (be a great way to delve more into her character from another person’s point of view and how’s she’s felt over the course of the series. I'd also accept a world playing as Kairi in Sora's mind.)
Agrabah/The Vanishing Isle - The Desert/Turtle Island
The Incredibles - Modern City/jungle island
The Emperor’s New Groove - Mountains/Grasslands/Rainforest
Then after KH4, with the seeding of it all, do a KHx/UX recap game/movie to help people understand those stories more, then focus on that in KH5.
I know that will never happen the way I want it too...but I can dream...
Love the idea of moving on from goofy and Donald to darkwing duck and max that be a real way to move the story forward. But live even more the idea of an axel spinoff (but id say they would do a 2 scenario side story bbs style with kairi being the second scenario if not xion)