War Doctor

This unknown incarnation of the Doctor was encountered by the Eleventh Doctor and Clara Oswald when they entered the Doctor's time stream on Trenzalore. (TV: The Name of the Doctor)

The Eleventh Doctor, upon coming into contact with this incarnation spurned the very memory of him, going so far as to suggest this incarnation did not even hold the moniker of "the Doctor" due to his actions in life. He explained to Clara that his real name was not the point; he chose the name 'the Doctor' and explained that the name a person chose was like a promise, and this unknown incarnation was "the one who broke the promise. He is my secret". 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".

This could hint towards The Doctor's actions in the Last Great Time War, and why he is named in the history Clara found in her exploration of the library. (TV: ''Journey to the Centre of the TARDIS).

Another theory is that Hurt will be playing the Valeyard as this is an incarnation of the doctor which is darkerand does not go by the monkier, "Doctor". This is further backed up by the fact that he does not seem to be an incarnation which preceeds the 11th, yet the 11th is still able to recognise him.