Toy Story World, I think is a pretty safe bet. Also, now with Disney buying almost everything (Marvel, Lucas Film even made a bid for Hasbro (Transformers)) there is a great chance that there will an Avenger world or Star Wars one, or both. They are 2 of the biggest fanchises in the movie industry right now like Pirates of the Carribean was back when KH2 was made. As for other worlds from the Disney-Pixar universe, I think they all have the same chance to make appearance.
PS A Lightsaber keyblade will look super cool. Also, it doesn't go any darker than a Sith as a villain.