Erm, did no one else notice that in the scene where Sirius dies, they're in a huge amphitheatre thing? I mean, I'm quite sure that Sirius is dead, but there must be a reason for that room to be in the Dept of Mysteries, mustn't there? You want my theory, Harry's going to go back to the Dept of Mysteries and there's going to be some kind of meeting of the dead in the amphitheatre in which Harry will be reunited with Sirius, if only for a brief time.