They shouldn't and wouldn't create a new story with a new hero. That's because if they did, so many more people would be against it than for it. KH couldn't survive with the storyline it has. They would have to change it for every new KH, and even though that seems to work with the FF series, none of the new things will seem to fit with the theme of KH that we have come to know. Only thing they will probably be able to do is create new Heartless and Nobodies. Sooner or later(most likely sooner) they will run out of Disney worlds, will move on to FF worlds, but as you see that won't work because then the stories of some worlds would be so screwed up if they were missing someone importan from it.(Like say Aeris,Cloud, and Sephiroth being missing from Midgar or coming there and saying they've never been there because they came from HB).