Unfortunately we might be waiting a while, if at all, because during an interview in the Ultimania book for Kingdom Hearts 3, the game’s director Tetsuya Nomura was asked about the possibility of seeing Marvel or Star Wars characters in the game. According to Nomura (as translated by KH3), that might prove difficult, if not impossible.
According to Nomura, “However, in order to change those [properties] into a game, contracts must be made with each company separately, and there are cases where other game companies already have contracts, so although the Disney Group has indeed added those [properties], incorporating them into KH isn’t so simple.”
He adds that even if Square Enix got permission, there are also complications where they might only get limited use of a character as another company might want to use the character as well. This isn’t to say that we might never see Marvel or Disney make their way into future Kingdom Hearts games, but don’t get your hopes up just yet.