I started to think that too. But I remember some of us thought the same for the 50th.What if Tennant is returning as 10.5?
And end up on Trenzalore and then in his final moments they put transfer the 10.5 Doctor's consciousness into the Library where River Song lives.What if Tennant is returning as 10.5? Maybe Rose died and he's found a way back to our universe? He acquires a Tardis somehow, finds a way to reverse the issue with Donna and then at the end of the specials he and Donna head off into the universe