Russell T Davies Had A 'Big Old Plan' For Peter Capaldi Long Before He Became The Twelfth Doctor

Peter Capaldi as Lucius Caecilius Iucundus in Doctor Who series 4 episode "The Fires of Pompeii"

All the way back in the ancient, forgotten days of September 2013, Steven Moffat, in an interview with Nerd3, had this to say about Peter Capaldi already having played two separate roles in two different TV shows set in the ever-contradicting Whoniverse:

"We are aware that Peter Capaldi’s played a part in Doctor Who before and we’re not going to ignore the fact. I’ll let you in on this. I remember Russell told me he had a big old plan as to why there were two Peter Capaldis in the Who universe, one in Pompeii and one in Torchwood. When I cast Peter, Russell got in touch to say how pleased he was, I said 'Okay, what was your theory and does it still work?' and he said 'Yes it does, here it is'. So I don’t know if we’ll get to it… we’ll play that one out over time. It’s actually quite neat".

Moffat, who certainly has his own fair share of big old plans (for instance, the return of Gallifrey), didn't necessarily rule out adapting Davies' plan into his own vision but, at the same time, it doesn't seem likely he'll tap it unless he finds a way for the idea (whatever it may be) to interlock well with his own Doctor Who continuity puzzle. He did, however, have a few more thoughts regarding how The Doctor's new face gets internally settled on during regenerations:

"The big fun question is, we know that the Doctor when he regenerates, the faces, it’s not set from birth, it’s not that he was always going to be one day Peter Capaldi. We know that’s the case because in The War Games [from The Second Doctor era] he has a choice of face and all that. We know it’s not set so where does he get those faces from? They can’t just be randomly generated because they’ve got lines and they’ve aged. When he turns into Peter he’ll actually have lines on his face (sorry Peter) so where did that face come from?"