I am not neccesarily in favor of this idea, BTW:
If there’s one thing that fans have come to expect from Doctor Who it’s that the cast will continue to change as the years go by. The Doctor’s companions come and go, and every few years The Doctor himself sheds his current look and (literally) becomes a new person. There’s always a lot of speculation about what form The Doctor will take with each upcoming regeneration, since every time The Doctor’s form and personality changes. Or does it?
Comments from Doctor Who showrunner Steven Moffat seem to suggest that The Doctor might slip back into an old familiar face once Peter Capaldi leaves the role. Instead of casting a new actor or actress to play The Doctor, the role might revert back to Matt Smith (who played the Eleventh Doctor before Capaldi took over the role).
According to The Mirror, Moffat has hinted that Smith might return to the show after Capaldi exits. Moffat also described Smith’s departure from the role three years ago, and how he wasn’t able to get the actor to stay; he described it as a tough time, saying “It felt like everything was blowing up around me.” Of course, Smith has since expressed regret for leaving the show when he did, and as Moffat put it is “quite open about how much he misses it, and how much he wishes he hadn’t left.”
Of course, Moffat’s hints at a possible return of Matt Smith doesn’t necessarily mean that we’ll see the actor waving his Sonic around anytime soon. So far we’ve never seen The Doctor regenerate into a form that he’s used before, though it was hinted in the 50th anniversary special that The Doctor would “revisit a few favorites” in his future. There’s also a precedent for familiar faces turning up in the show, as Capaldi himself was on the show previously (albeit as a different character) and had a brief identity crisis at the beginning of his run where he tried to figure out why he had that face.
I think it’s going to the well once too often. It smells of desperation and longing for the good old days when you were a “genius”. Like reliving past glories.
I could be wrong, but that’s just my opinion.