After introducing a new, previously unmentioned, Doctor last year, along with debuting Peter Capaldi in the episode prior to his formal introduction, some fans believe Missy to be a future version of the Doctor. Whilst this is probably something which the Moff would love to do (it has his name written all over it), due to actor availability is just isn't possible at all. Unless Peter Capaldi is departing at the end of this series (which is highly unlikely), Michelle Gomez cant hang around waiting for Capaldi to leave. Despite being given a new regeneration cycle, we doubt the Moff would waste one of these regenerations for only a handful of scenes. Ultimately what makes this one unlikely is the fact that Missy claims the Doctor is her boyfriend. Not even Doctor Who could pull off that timey-wimey incestuous relationship.
