"A life this long – do you understand what it is? It’s a battlefield, like this one… and it’s empty. Because everyone else has fallen."
The Doctor, Twice Upon a Time
The episode introduces the idea that a Time Lord might deliberately refuse to regenerate, and by so doing, succumb to whatever death has initiated the process. It also sharply reminds us of what the Doctor has lost over time, the manner in which both friends and foes have fallen by the wayside, finally leaving nothing but memories - a metaphor which is clearly illustrated by the avatars generated by the memory-harvesting Testimony, a ship created by scientists in the far distant future.
As Bill Potts' glassy avatar reflectively points out during the episode, letting go of the Doctor is hard. It's been fun watching Capaldi and the writers move his Doctor away from the stiff, formal character of his first appearance, adding sunglasses, hoodies and Doc Martins to his wardrobe, and the electric guitar* to his repertoire. (Not to mention his increasingly unruly hair as the series went on.)
It's been equally rewarding to admire Capaldi's skill as an actor. I've talked before about the partnership between writer and performer, and the writers' need to work with each new actor's specific talents while being faithful to the character of the Doctor. In Capaldi, the writers were presented with an experienced actor who had obviously developed his craft over time, and took full advantage of Capaldi's impressive ability to imbue his performance with emotion and power.
That ability was immediately evident in his first full episode, where his contemplations on the process of re-creation and replacement resonated strongly with the Doctor's own existence, and the writers were smart enough see what they'd been given and to take advantage of it. Capaldi's tenure was marked by resonant soliloquies and emotional monologues, or exchanges such as his epic, unforgettable rant on war from The Zygon Inversion in his second season.
Capaldi's final scene pays tribute to both his Doctor, and the Doctors who had come before him. He finally decides to give in to regeneration, and as he readies himself for the change, he delivers a reflective, touching retrospective on how the next Doctor should lead his life, followed by the long-awaited appearance of Jody Whittaker, whose first word as the Doctor is, quite aptly, "Brilliant..."
One could assume that the Doctor's youthful regeneration is an unconscious response to his decision not to end his existence, and perhaps this will be addressed in the new season. Sadly, we will have to wait a while for the answer to that question, or indeed any questions about the new Doctor. The BBC, after presenting the most anticipated regeneration in the history of the show, decided that they should wait until the fall of 2018 to start the new season of Doctor Who.
Wait, did we not just go through exactly the same process waiting for the 2017 season? (Because there wasn't one in 2016?) Coincidentally, Game of Thrones has also delayed their next season - until 2019 - and both shows are doing a reduced number of episodes. Ah, the confidence that all of these people have in the loyalty of their fandom - hopefully it's well placed. Careful, though: the Star Wars producers seem to be fully capable of popping out a movie once a year like clockwork, and it's a well known fact that the Dark Side has cookies.
Not a threat, just a warning...
*Which he comes by honestly - it's a well known part of Capaldi's biography that he was in a punk band called Dreamboys with Craig Ferguson (yes, that Craig Ferguson) circa 1980.