There are plenty of Disney worlds we haven't explored yet. The question is how far are they willing to go to do their homework to find them and make them work in KH.
I can list a bunch of worlds that could work in the next KH, like Frozen, Tangled, Wreak-It Ralph, Great Mouse Detective, Atlantis, Pocahontas, The Rescuers, Robin Hood, The Jungle Book, The Black Cauldron, The Incredibles, Toy Story, UP, Lilo and Stitch, A world based on Final Fantasy, and so many more.
Issues that they have to solve are whether these worlds are unique enough to appear, if their stories can "mesh" with the KH mythos, and if there is a conflict within the story that could foster heartless. It isn't as easy as you would believe to make the worlds "work" with KH.
For instance, Cinderella's world (Castle of Dreams) was one of the more complicated worlds to mesh, and had 2 separated bits. Ventus was made really small to interact with the mice and Cinderella in the house, while Aqua and Terra were human sized and interacted with people at normal height. They have to think of those kinds of issues when making the worlds.