We’re all expecting either Missy or perhaps John Simm’s Master to be hiding in the Vault, but what if it’s someone else?

What if it’s his granddaughter, Susan?

Susan in the Vault

We know from the Doctor’s desk in The Pilot that both Susan and River are on the Doctor’s mind, and we know Peter Capaldi would like to meet Susan. The trailer for Extremis gave us a brief glimpse of what looks like River Song’s diary, but I’m not so sure, though River can no doubt play the piano.

Of all the piece of the Doctor’s past, Susan (apart from in Big Finish) is the most open of companion departures. As a time lord, could she have regenerated, or perhaps the Chameleon Arch turn her into a human? Why keep her in the Vault? Perhaps it’s not a prison, but a protection, perhaps keeping her in existence as in our normal universe the Time War did something dreadful to her?

Unlikely yes, impossible? Let’s wait!