Ever since Disney bought Marvel, there's been those who do and those who don't want to see the Marvel superheroes join Kingdom Hearts. As a huge fan of any kind of crossover, I am for the idea.
So, I have two ideas, One, give the Marvel superheroes the same treatment as the Final Fantasy characters, having no real base world, have them be spread out and found in different worlds. Or, bring in a world based off of the Marvel Super Hero Squad, have the action taking place in Super Hero City & Villainville, like in the cartoons, and maybe Sora can even obtain a keyblade made from an infinity fractal to help power him up.
Also, with Oswald now apart of the Disney universe, I would like to see him become a part of the Kingdom Hearts franchise, and with that, I would also like to see a world based off of Epic Mickey's Waste Land, and while in Waste Land, Oswald could become an optional member of the team.