Put simply then, no one really knows who'll be occupying the TARDIS next season other than a handful of producers who recently confirmed that the role had already been cast. But fans widely expected that all would be revealed in this year's Christmas Special. Season ten concluded with Capaldi's Twelfth Doctor coming face-to-face with his original incarnation whilst attempting to fight off an impending regeneration. Many assumed that after the two Doctors teamed up this December, Capaldi's regeneration would finally take place, with the person replacing him likely being announced beforehand as is the custom.
However, it appears that may not necessarily be the case. On her Twitter account, director Rachel Talalay (who did a fantastic job behind the camera for the final two installments of season ten) claimed that she had "no knowledge of 13." This comes despite Talalay returning as director for the Christmas Special. You can see her post in the space below:
To answer general q's: No trip to SDCC for me this year. No knowledge of 13. No idea about S11. Reading some books. Any recommendations?
— Rachel Talalay (@rtalalay) July 11, 2017
This news could mean a variety of things. Firstly, it's possible that the identity of the thirteenth Doctor won't actually be revealed this Christmas and that the episode would simply end with Capaldi starting to regenerate, leaving the eventual reveal for the season eleven premiere. Alternatively, despite the Christmas episode itself having already wrapped filming, the end product of Capaldi's regeneration may be shot at a later date but still included in the Christmas episode. Lastly, it could simply be that the director is sworn to secrecy on the matter and is unable to reveal any information.
If the Doctor's new face wasn't to be revealed in the 2017 Christmas Special, it's likely many fans will be left somewhat disappointed. Capaldi's departure has been known for some time now and with his regeneration in progress, December already poses a painfully long wait for Whovians.
With that said, the constant cat-and-mouse back and forth over who will play the next Doctor is all part of the fun of loving the show and the more teases, denials and confirmations fans get, the more their anticipation for the eventual reveal grows. It's all part of the Doctor Who experience.
The Doctor Who Christmas Special airs December 25th on BBC.
Source: Rachel Talalay (via Twitter)