I think it's possible, but not necessary. The thing is that they would have to find a use for Disney Castle/Disney Town in the game. This could be done in a variety of ways:
1.If Maleficent is planning on stealing the Datascape this time around, we're bound to be in the castle for at least a cutscene.
2. If Disney Town becomes a minigame world where you can play all of the BbS games in higher quality and you first get classic kingdom (not likely).
3. If Disney Castle is pursued by Xehanort or any other opposing sides, we're bound to see it.
Here's the problem with the last one: why would they attack it? The group's hub world is clearly the mysterious tower, with every known wielder there. If they're going to be stashing princesses at any point to keep them out of harm's way, they would probably do it in that dimension that Merlin opened up.
Disney Castle should be the hub world, I agree. That would be awesome. The problem is that I don't see much of a use for it in the game. I think it's likely to show up in some form during the game, whether it's just seeing Chip and Dale interact with it quick or as an aside to the datascape since Data is going to be included in Big Hero 6, but seeing it as a full world? Probably not, I just don't see what they would do in it.
Oh, and to the original or Disney question, I'd say Disney if not for the fact that Timeless River is based on Steamboat Willie and is technically the same world. It is an original design, but there's so much licensed property in it that I doubt you could make a case for the world being wholly original.