Here's a bit of fanwanky speculation. No spoilers, apart from one guest actor. None of this will happen probably, but eh, what else are we going to talk about.
So, one of the guest actors next season is going to be Struan Rodger. He already appeared in the show before as the voice of the Face of Boe, but I'll ignore that for now. A friend pointed out that he looks remarkably like the first Doctor in Game of Thrones:
Sure, sure, we already had David Bradley playing the First Doctor during the 50th anniversary, and he'd probably be the first choice if they ever tried to bring the First Doctor back for something. And, more importantly, they'll probably won't bring back the First Doctor at all, especially so soon after the last multidoctor special.
Except, what if they do? Moffat certainly isn't scared to go deep into the history of the Doctor (as evidenced by Clara showing up under the bed of the youngest version of the Doctor we've ever seen last season), and we still have this little unresolved storyline.
Peter Capaldi as Caecilius in Fires of Pompeii
Peter Capaldi as John Frobisher in Torchwood: Children of Earth (Frobisher was also the name of the Sixth Doctor's shapeshifting pinguin companion, but I won't go down that rabbit hole for now - although a shapeshifting pinguin being behind it all would explain everything)
So yeah, Peter Capaldi showed up in the show before he became the Doctor. Could be something that's easily ignored, but we already had the Doctor wondering where he had seen his own face before a couple of times last season. And this isn't the first time characters looking identical to a certain Doctor have showed up in the show. Something that, again, could be easily ignored, but could also be mined for weird plots.
Colin Baker playing Maxil, a Galifrayan Guard Commander in Arc of Infinity, before he got the role as the Doctor.
Patrick Throughton as Ramón Salamander in Enemy of the World, opposite himself as the Second Doctor
Other Timelords who went through the same thing:
Lalla Ward as Princess Astra in The Armageddon Factor, before she was cast as the second incarnation of Romana. This one was actually explained in-universe by showing that Romana somehow has more control over her regenerations, and she specifically decides to model herself after Astra.
Anthony Ainley as Tremas in the Keeper of Traken, before he became the Master. This one was explained as well as the Master specifically stealing that body. He later pulls more or less the same trick in the TV Movie with McGann
Of course, there are many more instances of actors who played the Doctor playing other characters over the years in the audio's and some fanfilms from the 90s, but I won't go into those for the sake of sanity.
The one that I wanted to talk about when I started writing this post:
William Hartnell as the Abbot of Amboise in The Massacre of St Bartholomew's Eve. The Doctor and the Abbot never actually come face to face in this story (they save that little trick for Enemy of the World).
My prediction: Struan Rodger won't be playing the First Doctor in the next series, but he will be playing the Abbot of Amboise, setting in motion a series of events in which the Doctor tries to find out why there are duplicates of himself scattered around time.
None of this will happen, but if it does, I demand some sort of prize.