Russell T Davies says bi-generation will have huge implications, and it sounds like we’ve got a multiverse on our hands

gamesradar.com:

Doctor Who’s final anniversary special, 'The Giggle', introduced the concept of 'bi-generation', and it’s thrown the whole Whoniverse into chaos. Now, we’re going to get into spoiler specifics below, so make sure you’re up to date on all things Who before going any further.

As the regeneration came around, it turned out that the Doctor could now split into two, rather than simply regenerating into a new version of themselves. This means that David Tennant’s 14th Doctor and Ncuti Gatwa’s 15th Doctor can now both exist in the Doctor Who universe.

But it doesn’t just mean that as showrunner Russell T. Davies has confirmed the implications are much, much bigger. In the commentary to the episode, he explained that "the whole timeline bi-generated" when this moment happened, meaning it’s not just Tennant’s version that was impacted. 

"Sylvester McCoy woke up in a drawer, in a morgue, in San Francisco and Jon Pertwee woke up on the floor of the laboratory," he laughed. Meanwhile, producer Phil Collinson suggested: "Colin Baker got up and sorted the Rani out." Davies also suggested that Peter Capaldi's Doctor and Tennant's previous Doctors also woke up in the TARDIS and are now "out there" as well.

Davies also said earlier in the commentary (at the 12:30 mark if you're interested): "I think all of the Doctors came back to life with their individual TARDISes, the gift of the Toymaker, and they're all out there traveling round in what I'm calling a Doctorverse."

The comments have already got fans speculating that there may be some big spin-offs on the way as a result as we deal with what is now a Doctor Who multiverse. 

And it certainly seems at least that Davies is open to more. Per Radio Times, during the Doctor Who Unleashed