In a revelation that will please everyone (and surprise no one, given just how much Disney owns), Kingdom Hearts creator Tetsuya Nomura has announced that he wants to include characters from Marvel and Star Wars in the trilogy's final installment.
"Of course it would be great if we could add [the Marvel & Star Wars] licenses as well..." Nomura said at the Japan Expo 2013. However, wanting to do something and actually being able to are two very different things - a fact that Nomura also recognises:
"... there are lots of rules and restrictions by Disney so we can’t actually put in everything people want. So it’s a quite difficult decision but we’re gonna prepare some surprises for you so, just wait for the information.”
So, this isn't a definite confirmation - but they'd have to be mad not to do it. A game crossing over this many things would probably be a great money maker.