Well, I wouldn't mind seeing the Kingdom Hearts saga contintueing after the Xehanort saga, and contintueing as far as the Final Fantasy games. But there are problems with it continueing father though. There are alot of Disney movies out there, but only a few them can actually be made part of the videogame. Of course, they could make more original towns/worlds, like Twilight Town, Radiant Garden (or Hollow Bastion), and the World That Never Was. They can also make more original characters. But with Final Fantasy continueing as long as it has, is because in every game, its a new story with different characters. (Excluding FFX-2 and FFXIII-2 and the movies/other prequel games to FFVII).
And, if you watch the secret ending to KHBBS, Master Xehanort states that Terra was only one of many roads he might choose to take and that he made certain of that. There are other people who side with darkness, the game might not show them at the moment, but they could be there.
Also, the saga after Xehanort might revolve around Sora connecting the worlds back together again. I kind of doubt that though considering their seperated for the reason of there being selfish people will take the light.
And, just because the Organization(excluding Roxas who isn't even part of the Org. anymore) is destroyed doesn't mean there are no more Nobodies. And I'm not talking about Dusks, or Dancers, or Assassins.