The Timeless Child retcon promises to transform the Timeless Child, the base genetic code for the entire Time Lord race.
This dramatically rewrites the history of the Doctor, revealing she is older than Gallifreyan civilization itself - meaning she is over a billion years old, and there have been countless incarnations of the Doctor viewers haven't seen. Only a handful of these have been seen; at least seven who were experimented upon by the ancient Time Lord scientist and explorer Tecteun, those glimpsed in the classic Who story "The Brain of Morbius," and possibly Jo Martin's so-called "Forgotten Doctor." Audiences are divided over the Timeless Child retcon, with some hoping Chibnall will retcon it as nothing more than a lie, while others are willing to wait to see how Chibnall plans to build on this foundation.
The BBC has actually dropped a subtle hint they will indeed be exploring the Timeless Child, who was teased as having been found at the foot of a mysterious "boundary" that may well lead to another universe. Back in January, when the BBC announced new companion John Bishop as a character who has simply been named Dan, they teased he would "quickly learn there’s more to the Universe(s) than he could ever believe." It's an intriguing comment, because while Doctor Who has strayed beyond the confines of this universe before now - with Tom Baker's Fourth Doctor spending an entire season in a pocket universe called E-Space - that's never been seen as the main marketing hook before.
The clear implication is Doctor Who season 13 will take the Doctor beyond the boundaries of this universe, as she presumably learns more about her origin. Having revealed the Doctor is not actually a Time Lord, presumably Chibnall intends to use season 13 to reveal just what she really is. Modern viewers are about to experience the same sense of discovery older viewers got from 1969 onwards when Doctor Who first introduced the Time Lords. What's more, this should give season 13 the emotional arc it needs, as the Doctor is forced to learn the truth of her identity. This means season 13 has the potential to expand the franchise on an unprecedented scale. The Doctor has always been able to travel throughout time and space, from the Big Bang to the Big Crunch, but now she is exploring a whole new universe too. Even more intriguing, this universe is likely to be populated with races and beings never heard of before, meaning the writers have an unprecedented creative opportunity.
No doubt many viewers will be approaching Doctor Who season 13 with a degree of skepticism. The divided reaction to the Timeless Child reaction was in part a reaction to the flawed writing, with the narrative actually attempting to implement a radical change to Doctor Who's status quo, while at the same time trying to tell viewers it didn't really matter that much. The problem was as much in the execution as in the idea, and Doctor Who season 13 will need a lot stronger writing if it is to make this major retcon worthwhile.