They have already announced a third Kingdom Hearts title, a few years ago in fact (If you don't count Kingdom Hearts 2 or the three new handheld versions). It's supposed to come out on PS3 I think
As for including Marvel characters, that won't ever happen. The kingdom hearts series is known as a very fantastic fantasy realm, while marvel comics are somewhat graphic and very serious. Any attempt to include the comic book heroes will dis-credit both the comics and the game.