Hmmmm... Thanos wants the keyblade maybe? And the Emperor and Thanos team up with other Disney villains? I'd like to see a larger scale for the KH series, a big universe to explore... touching Avengers for Marvel, Era of the Empire for Star Wars, maybe leave Indy out of this its so hard for me to keep up the energy to block out the fact that there was a fourth movie (NO THERE WASN'T).
But expand on KH with the big names and... ooohhh just thought about seeing a Gummi ship fight with a Gummi-Star Destroyer and Gummi-TIE's, followed by landing on the incomplete Death Star II orbiting Endor. Escaping the Death Star in a Gummi-Millenium Falcon? Or charging headlong down into the DS with the Falcon and Gummi-X-Wings to take it out...
Ok I'm sold, I don't care about bloat, it would be fun and it would work.
Avengers? Hell you could visit Asgard, Earth, and clash with Thanos somewhere along the way if they want to go that route, or maybe just Loki... still would be interesting to me.
But hey, I like Kingdom Hearts specifically for its idea that all the worlds are connected somehow, and it works for me. But I don't have super high standards so take this as you will.
