I want to see Toy Story in it and Sora, Donal and Goofy in toy form. I imagine a Toy Story world could provide endless possibilities for minigames too.
As for FF characters it'd be cool to see moombas, Laguna or any other of the as yet excluded FF8 main cast, especially Rinoa.
Also since the hearts of worlds is like the lifestream of KH, then put in the Shin-Ra company to suck it up like mako. And just as they were after the promised land in FF7, they could seek Kingdom Hearts and you have a story.
