As was just revealed at the Doctor Who panel at New York Comic Con, in addition to the Doctor and Matt Lucas' character Nardole, the Christmas special — entitled "Return of Doctor Mysterio" — also features Justin Chatwin (Orphan Black) as a literal superhero. Here's the full image of Chatwin:
— Doctor Who Official (@bbcdoctorwho) October 7, 2016
Steven Moffat said that his inspiration for the special was his obsession with superheroes, in particular Clark Kent. Not Superman, but Clark Kent, because Moffat loved the mild-mannered alter ego. In particular, the way that he never tells Lois Lane that he is actually the man she loves. "It's a love triangle with two people in it!" It is, in his mind, the best love story ever told.
We also saw some of the very first footage from the special in a behind-the-scenes video. Be prepared for a lot of weird things confusing the Doctor: