3) The Show Has Been Building Up To It
One of the best things that Moffat has given us – and that he isn’t credited enough for doing – is creating the idea that Time Lords can switch genders as part of canon. The idea was first floated in 2011’s “The Doctor’s Wife,” written by Neil Gaiman and then kept gaining traction from there. By the time Peter Capaldi’s version came around, it was rising to the forefront of the show.
Though he’d cast Capaldi in the lead role, Steven Moffat recognized the call from fans for a female Doctor and decided to test whether the wider audience as a whole would accept the idea of a Time Lord becoming a Time Lady in a different way first. In Capaldi’s initial series, he squared the Doctor against Missy, as played by Michelle Gomez. In other words, the Doctor’s arch-nemesis the Master in female form.
The ‘test run’ went down a treat and Missy is now one of the most popular incarnations of the Master ever. After that, he had the Doctor hinting that he wouldn’t mind being a woman in 2014’s “Death in Heaven” and the first on-screen male-to-female regeneration in 2015’s “Hell Bent.” The recent season 10 finale, “The Doctor Falls” was full of references to Time Lords becoming women, as well, with the Doctor commenting that he doesn’t know if he has been a woman in the past and hoping that “the future would be all girl.”
In short, the introduction of a female Doctor isn’t something that has come out of the blue. In fact, the show has been cleverly and quietly building up to a place where we could accept a female lead for some time now.