Well Dean's books and Billie were wrong again weren't they? You would have thought that one of them might have showed Jack killing Michael and saving Dean, unless Micahel's not really dead?

My money (unfortunately) is on Lucifer coming back and maybe teaming up with Jack. Although I'm not sure how we'll get there unless the gang get super freaked out by Jack again. I really hope there's something more exciting on the horizon than that (I'd take anything over Lucifer at this point!)