It's a show that uses Time travel as it's primary plot device, they can bring back any Doctor they want (like say the current rumour of the Christmas special being 12 and 1), every incarnation of the Doctor has enough unaccounted for time to squeeze in extra adventures if they wanted to bring someone back.
But outside of a one off special, it's not going to happen.-
But outside of a one off special, it's not going to happen.-