The Regeneration Theory That Would Change Everything On Doctor Who
There are very few sci-fi franchises out there that can rival "Doctor Who" and its nearly 60-year-long history. The British series, which has centered upon the mysterious time-traveling alien the Doctor and his companions since its very first episode in 1963, is nothing short of a cultural icon. Audiences with even just a passing knowledge of the show are aware of its hallmarks, such as the signature blue police box form of the TARDIS time machine, the evil bucket-shaped Daleks, and the Doctor's uncanny ability to change faces every couple of years or so.
Indeed, part of why the show has been able to run for so long while focusing on the same central character owes to a mechanic introduced early on: regeneration. Essentially, due to his biology as a Time Lord, the Doctor does not die when he sustains a grievous injury but actually adopts a brand new face and body, along with a new personality. This ability allows a new actor to inherit the role and continue forth in the show without the usual challenges of recasting a character. Though regeneration is a bizarre concept on its face, it has allowed actors like William Hartnell, David Tennant, and Jodie Whittaker to all play renditions of the same character. However, there is one popular theory in the "Doctor Who" fan community that could shed even more light on how this mysterious process functions in-universe.
The Doctor regenerates into faces he's already seen
A fan theory posted by Reddit user u/theklinik on the r/DoctorWho subreddit suggests that the Doctor subconsciously regenerates into the form of people he's encountered in the past. As the theory recounts, The Doctor has chronologically encountered his future selves on many occasions, such as when the First, Second, Third, and Fifth Doctors all met in the episode "The Five Doctors." Though the show has established that only the latest iteration remembers multi-Doctor events, it could be the case that the younger Doctors subconsciously chose their future selves' faces when they eventually regenerated.
Perhaps the most credible piece of evidence, however, was when Matt Smith's Eleventh Doctor regenerated as Peter Capaldi's Twelfth Doctor. Capaldi had previously appeared as a Roman man named Lobus Caecilius, who was saved from Pompeii's destruction by David Tennant's Tenth Doctor. The Twelfth Doctor realized early on that he had subconsciously chosen the face of Caecilius as a reminder to himself that it was his duty to save people. This was arguably an outright confirmation of the theory, though it was only explored in the context of a single regeneration.
While this "Doctor Who" theory hasn't yet been proven beyond Capaldi, it could be true for the Time Lord's other past lives. It's certainly an exciting proposition for fans who like to speculate about the future. Perhaps viewers have already seen the face of the Fourteenth Doctor on the show and they just don't know it yet.