In the 50th special, we see Tom Baker as the curator of the museum, having an odd conversation with Matt Smith at the end, where Baker’s similarity to the Doctor’s earlier incarnation is heavily alluded to.
It’s odd, and I had chalked it up to bad writing for the sake of making some jokes. It didn’t make sense that he would be visiting himself in Baker’s incarnation as an old man, as he never got that old to my knowledge.
But one thing the Curator says is that in the future, the Doctor will be seeing some old, familiar faces.
Then, in Deep Breath, Peter Capaldi implies that he gets his new faces from somewhere. In one sense, it’s to explain the fact that Peter Capaldi has been in Doctor Who before (as well as Torchwood).
But what if it is a way of getting former Doctors to make cameos? If every regeneration gets the appearance from a real, other person, then all the actors portraying the Doctor can come back as that original person, just aged.