Or – and this was my reading of it – it's Moffat laying the first hint at this:
We are aware that Peter Capaldi’s played a big old part in Doctor Who before and we’re not going to ignore the fact… and Torchwood. I’ll let you in on this. I remember Russell [T Davies] told me he had a big old plan as to why there were two Peter Capaldis in the Who universe, one in Pompei and one in Torchwood. When I cast Peter and [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.
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 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?
Along with that whole bit about the Doctor 'going home' to Gallifrey, it seemed like the whole last ten minutes were all about seeding the future story arcs post-Smith.