In those serial-killer detective books, all the clues that you need are there at the first crime scene and I think that the same principle clearly applies in trying to establish what the season-long story arc will be: Exhibit A m'lud, is a monster who has made and remade himself, generation after generation, an individual who seems to have lost sight of what he was supposed to be in the first place.
In philosophy, this is the problem of Theseus' ship:
“Theseus owns a sailing ship. One day he sets sail on a long voyage.During this voyage Theseus anticipates his ship may need various repairs this and so he brought with him a complete supply of new parts to make repairs on his voyage. As repairs are needed, Theseus throws the old parts overboard and replaces them with new parts.
Shortly out of port, Theseus notes a plank needs replacing. He replaces the plank, throwing the old plank over board. Now is this still the same boat? Surely it still is?
As Theseus is approaching the end of his journey, he has replaced all but one of the planks, now there is only one original plank remaining form the original ship. Is the ship the same ship that set out from port?
Finally Theseus replaces the final plank. Now Theseus has completely rebuilt the ship. He returns home to Greece (or wherever it was) after defeating the Minotaur (or whatever he did) in this completely new ship.
So the question is: “is this ship at the end of his journey the same as the ship that began the journey?” (link)So, it seems likely that this will be a season of ontological crisis for the Doctor : What is he and what has he become? This is hardly a novel insight - he's practically said as much in the trailers. (I don't know what Moffat or Capaldi or others have been saying in interviews - I did very well in keeping away from spoilers this year)
I also guess, given that little 'Promised Land' Marvel-post-credits-style scene at the end, that there will be some religious overtone to this debate. We shall see. The Daleks are up next week, and they've been acting like a fanatical cult for ages now, so I'm sure we'll see the question move on.
It's nice to have the question of the true nature of the Doctor feel so wide open. I really thought that the Doctor had pushed that fleshy robot fellow onto Big Ben at the end, but other folk online seem to think that in fact he jumped*. Either way, everyone who's seen Daniel Craig in that 'Enduring Love' film knows that, if you allow folk to fall from hot air balloons in a morally ambiguous context, you are only storing up trouble for yourself in the long run.
(*I do know, of course, that there are no such things as 'facts' in such a narrative context - but the whole business is less fun if you observe this rule consistently)