ok after looking over some stuff I realize I made some errors in what I said.
Sora is not from the FF dimension.
He simply lost his world and reappeared in Traverse Town like how most other FF characters survived. Each have their own world, even if they were destroyed.
The only strong relation to the FF characters is that each of Sora's home bases are also where most of the FF characters also live, Traverse Town, Radiant Garden (Hollow Bastion). These worlds were created by Nomura specifically for the game, so, I was wrong in thinking that this made them related.
Secondly, I didn't see where it said there would not be FF characters in the game, just that there wouldn't be any playable ones.
Third, I forgot that KH has a lot of S-E developed characters which are not Disney. When I read the OP I immediately thought that they were getting rid of all these characters as well and instead focusing only on disney characters. As such, I think the game can survive without the FF characters because the other S-E characters adequately convey enough SE influence.
Lastly I just want to reiterate my point.
FF was never hugely important to the storylines, but they did act as transitional plot devices. They also add some great atmosphere, reflected by Nomura when he said that Disney characters are not dark enough. They simply contribute to the atmosphere of the game.
Changing anything about a successful game demands inspection and criticism where it applies. History has shown that this game is successful with the FF characters.