That wooden door I believe is the keyhole to DI. But besides that....yeah, of course we'll go back. And just because Sora is trying to get there doesn't mean it will be the last world. He could get there, almost stay, but then Kairi(even though regretingly) tells him he still has a job to do, and then they have a scene where they kiss goodbye and he promises her he'll come back soon. Or, what could also happen, is that they go back and she joins up with them there or may come to other worlds with him("This time, I'll fight." Hmm, remember that quote?).