I think that, at some point, if SE wants Kingdom Hearts to continue to be one of their main pillars, they're going to have to ditch Disney. I could go into a longwinded explanation about that, but I don't want to. Basically, you can only use the same characters so much before they lose their novelty. The same might apply to Final Fantasy characters, but because they're not linked to their plotlines from the games, there's more potential. They've just taken the character and fit them in wherever it works. There's many more characters that they can do that to.

Personally, I feel Kingdom Hearts is at its best when you're in original worlds. The majority of the plot is also revealed in these worlds. The Disney worlds, for the most part, become filler between the major plot developments. This is especially apparent in Kingdom Hearts 2, although they did do a better job bringing some of the main plot into the Disney worlds in the second half of the game.

I await the day when Kingdom Hearts resigns Disney characters to the same fate as Final Fantasy characters: cameos. I think this would allow for a more fluent plot development instead of throwing it at you in chunks as well as opening new opportunities to the Kingdom Hearts series.