Additional out-there Who theory

[yendi]

Yes, I still prefer the theory that the new Big Bad is really the Master, but given the frequent use of mirrors, what if it was Daughter of Mine? Or possibly Mother of Mine, with the rest of the Family being called in?

Yeah, I know that's not likely at all (how often does anything from the Davies run get mentioned unless Moffat wrote it?), but it's another group of antagonists who can change appearances, and it's the one group from the Modern Who run who have encountered The Doctor at his darkest and most vindictive.