How the fudge are WE supposed to know? We don't even Know all the worlds that will be in KH2. But to sum it down to the ones we DO know.
Deep Dive/ASAS City- This place seems like an interesting place to end a game. And the fact that it was in both KH1 and Final Mix,it MUST mean something important will happen in this place. And its dark background seems to fit KH2's pesona perfectly.
Destiny Islands- Quite possible, this is a place where Sora's story began. Why not end it here? But in KH1 destiny Islands/End of the World was the final level. I have my doubts as of to whether Square would reuse the same world for a final level in a sequel. But most likely the story will end here for a happy ending. If not,prepare for a KH3, 4-5 years from now.
Hollow Bastion: I REALLY don't think this will be the last level. We've seen both trailers and screenshots from this world.It would be a good place to end the game,but Square would NEVER show us anything retaining to a last level.It would spoil EVERYTHING. We've seen a bit too much for this world to be the final.
Or it could be some new world that we don't know about yet. Only time will tell. But I'm thinking its a world that is currently unannounced at this time. So,what do the rest of you think?