There was a throwaway line in the 50th Anniversary Special by the Curator that could shed some light on this. He mentions, in sequence, that when the Doctor speaks on retiring to be the Curator, "You might", and when speaking about never forgetting faces, "You may find yourself revisiting some of them again."
That speaks to me of a new set of regenerations, where the Doctor chooses to go back to the looks of previous regenerations. Quite a nice way to give former Doctors who want to return some more screen time.