War Doctor
An unknown incarnation of the Doctor was encountered by the Eleventh Doctor and Clara Oswald when they entered the Doctor's own timestream. The Eleventh Doctor hinted that this incarnation of the Time Lord was responsible for a transgression that dishonored his name.
This unknown incarnation responded by defending those actions, stating he had "no choice" and that he did what he did "in the name of peace and sanity."(TV: The Name of the Doctor)
Behind the scenes
Assuming this Doctor is an actual incarnation of the Doctor (and not a copy or a manifestation of some part of the Doctor's personality), then Hurt, aged 73 when he first appeared on screen, will become the oldest actor to portray the Doctor upon their debut appearance. The second oldest would be William Hartnell, who was 55 years old when the show premiered in 1963.
