The fact that the new Doctor's TARDIS is a Police box suggests that she cant be pre-Hartnell, especially when you consider that we saw the Hartnell Doctor leaving Gallifrey when Clara encountered him. Some have speculated that she could come between Troughton and Pertwee although that would mean she changed back into Troughton's clothes before regenerating! And of course Troughton had a sonic screwdriver, although it was quite different from what came later.
A lot of people are going with a Doctor from a parallel universe but Chris Chibnall has this to say:
“The important thing to say is – she is definitively the Doctor,” he explained. “There's not a sort of parallel universe going on, there's no tricks. Jo Martin is the Doctor, that's why we gave her the credit at the end which all new Doctors have the first time you see them. John Hurt got that credit.”
Chibnall also says that Captain Jack categorically will not be back this series buts some fans have leapt on the fact that behind the scenes footage shows that his scenes were directed not by Nida Manzoor but by the director of episodes nine and ten. This doesn't necessarily mean that he appears in those episodes though, it might simply have been dictated by his availability.
Edited by Robbie Moubert on 28 January 2020 at 9:10am
My theory is that Brain of Morbius is coming back into continuity. My theory is that the Time Lords did something terrible or had something terrible happen, and based on that they erased their memories of it and rewrote their history to hide it. So those Brain of Morbius faces were the Doctor. Jo Martin was the Doctor. There were probably any number of other Masters, etc. as well. That's what the Master discovered, the real history including his real history, and that's why he blew everything up.
This might also be when they created all the rules. Limited regenerations. Non-interference. No changing the timeline. No meeting yourself. There's a good case to be made that those rules are just artificial since the Doctor has broken them all the time without much consequence other than the anger of the Time Lords.
Edited by Steve De Young on 28 January 2020 at 6:20pm
Chibnall continues his run of "The only canon that matters is mine"
Weird comment on an episode featuring two RTD throwbacks. And if you are referring to regenerations, arguably this is a return to previous canon, as opposed to something that's only been in place since 2009.