We keep hearing rumors, but there's just not a lot that's going to fit into a 60 minute special.
I understand there's some kind of funding issue that leaves the series future in doubt, but they would have done better to hedge their bet for an anniversary series 8 where each episode could focus on something related to one past Doctor (13 episodes - 11 Doctors with two bookends to start and finish the series). I am really shocked this is not what was done; I don't think Moffat would be opposed to a grand epic season of Doctor Who history even if it was after the 50th year. What better story would he have to tell for Series 8?