The 13th Doctor, played by Jodie Whittaker and helmed by showrunner Chris Chibnall, is the gamechanger for Doctor Who in many ways. First, she was the first female lead in canon. Then in her second season, Chibnall dropped a bombshell. The Doctor was the original Timeless Child, an immortal being from an unknown place who became the basis for all the Time Lords.
The child came to the universe through a gateway from another universe or dimension. Her adopted mother Tecteun discovered her regenerative ability to turn into another body and even gender if she suffered a fatal injury. Tecteun experimented on the child to understand regeneration, then gave it to herself and the elite of Gallifreyan society. Now immortal, the Gallifreyans developed time travel technology and built up the planet with the new Time Lords the ruling elite. Every Time Lord had some of the Doctor's DNA in them. Without the Doctor, there would be no Time Lords. Chibnall said "The Doctor is non-binary" a year before this retcon. He was planning this from the start of his run.
The Retcon That Changed Everything
This is one of those "everything you thought you knew was wrong" retcons that writers introduce to spice up a series that threatens to get long in the tooth. Of course, it came out of comics. Alan Moore's deconstructionist approach casts a long shadow over this approach as the current showrunners were all influenced by his work. Russell T. Davies did it by having Gallifrey destroyed and making The Doctor the one who did it to stop The Time War. Then Davies brought Gallifrey back at the end of his tenure. Steven Moffat did it by changing history so the Doctor didn't destroy Gallifrey at all but preserved it. Now Chris Chibnall takes it further by not only destroying Gallifrey again and now revealing the "truth" behind the Doctor's origin and the nature of regeneration and Time Lord lore.
The Doctor Was an Abused Child
Chibnall's change to history also answers some fundamental questions that have lingered over the show's entire history. He turned the Doctor into an abused child at heart and Gallifrey her abuser. It explains why the Doctor may save Gallifrey time and again, but never wants to stay. The Doctor always gets the hell away once that good deed is done. Chibnall doesn't spell it out, but that's because the Doctor can't bear to stay at the place where they abused her for thousands, possibly millions of years. They may have wiped her memories, but deep down, the Doctor remembers. The question of why The Doctor ran away is answered at last. Even after The Doctor gets elected President, he never stayed to enjoy the power and prestige, because he knew it was founded on lies, hypocrisy, and her exploitation.
Seeing "Doctor Who" in a New Light
The Doctor's abuse at the hands of Gallifrey also explains why the Doctor became a protector of children. It's not just the right and kind thing to do, it comes from a place of trauma and deep empathy. Granted, in a show for children, the hero doesn't really need a reason to be a friend and protector of children. Now Chibnall has given the Doctor an emotional reason for doing it.
To watch the old episodes of the show before Chibnall's retcon actually gives them an additional layer of meaning now. It doesn't diminish the past stories at all. When you watch past versions played by William Hartnell, Patrick Troughton, Jon Pertwee, Tom Baker, Peter Davison, Colin Baker, Paul McGann, and Sylvester McCoy and their deeply ambivalent interactions with Gallifrey takes on new meaning knowing their origin as the Timeless Child. Chibnall has also introduced more unknown incarnations out there prior to the First Doctor that opens up more story possibilities, not less. Chibnall has made our hero the underdog again, and more mysterious than ever. With unlimited regenerations, the show can also now go on forever with no end.
I suspect Chibnall has also embedded the endpoint for the Doctor's story here. If he chooses to do it, the Doctor's final story will probably end with her dying and going through a gate, only to end up in the distant past and regenerate into the original Timeless Child to be found by Tecteun. That's my guess. Of course, a new showrunner could always retcon this all out in a flash and we'll be off on another path. But that's the nature of Doctor Who. It's a show whose own continuity has always been fluid. Writer Paul Cornell once said that the universe retcons itself every time the Doctor gets in the TARDIS and dematerializes. Don't like this bit of canon, don't worry. A new one will be along in a moment.