From the BBC America Tumblr:

Keeley Hawes (Line of Duty, Upstairs Downstairs) will join the cast of Doctor Who, starring opposite the Doctor. Hawes will play Ms. Delphox, a powerful out of this world character with a dark secret. Traveling across space and time, the Twelfth Doctor (Peter Capaldi) and his companion, Clara Oswald (Jenna Coleman), will come face to face with the mysterious Ms. Delphox when they arrive on a strange and puzzling planet.


So we commence mad speculation.......ALTERNATIVE DIMENSION DOCTOR!!