I could see how characters from KH could easily be added to Fantasmic, or how it could be used to replace the current show. The villians do have at least one scene in the game (still playing it for the first time) where they are all together, and where they plot together, and the good guys are a kid, Goofy and Donald, with movie "stars" helping them out on different stages. It sets up into a good good vs. evil story, much as Fantasmic already is.
I think kids might take to a KH show very well. Instead of having Mickey as the lead, you'd have the main character Sora, who I think is like 13 in the first game. If Disney is about imagination, children would have an easier time picturing themselves bannishing evil as Sora instead of as Mickey.
If Disney wanted to freshen up the show, and I'm not saying they should, this would be an easy, quick and cheap way to do it. You could do it by adding a man dressed in overalls and carrying a large sword-like weapon. and maybe add some of the evil characters who could just be people in all black.