Castle Oblivion/Twilight Town. If you remember, you got Riku into Castle Oblivion by going through a room in Twilight Town... My guess is Sora, Donald, and Goofy wake up in Castle Oblivion and wander on in to Twilight Town, meanwhile events with Blonde Haired Kid and his friends are taking place like how things were happening on Destiny Islands and Disney Castle at the beginning of Kingdom Hearts. They will have to make a tutorial on the controls again at the beginning for new players and for old players as they tweaked the game up some. (This could possibly be a chance where you get to play as Blonde Haired Kid. Not confirmed that you play as him, but there's a chance...)
