It is the combination that sells so well. Both disney and square. They will soon run out of new worlds to head to though, I highly doubt they would create ff worlds. I believe the next kingdom hearts will be like a look into the past, or traveling time, or something to the similar effect. It will probably be on the PSP, and the one after this will be KH3 where they return to Sora. Or they might introduce a new character and just have Sora and Co as cameos. Whatever is done they will never have entire ff worlds, although I believe a villain other than Sephiroth may be seen next time (either Seymour or Edea/Ultimecia) as characters from both 8 and 10 have been introduced, and the fact that the letter Leon receives is a high candidate for Rinoa(see threat Leon's Letter), a sorceress might not be too far behind.