Ya know, the thing about the Timeless Child retcon in Doctor Who is that it was actually a good idea overall, it just shouldn't have been the Doctor.
Even back in the classic series, the Time Lords were known to be corrupt and almost evil. In the new series, they were talked about positively, but only because the Doctor was talking about a people who were all killed off. As soon as they found out that the Time Lords might come back, the Doctor literally grabs a gun and rushed as fast as they can to prevent that from happening.
But the most we ever actually SEE the Time Lords do is just be assholes. They're jerks. We hear that they did terrible things during the Time War, but any time we've seen them they're just asshole politicians talking shit about the Doctor while also putting them in charge of the planet. (I actually think it would have gone a long way if there was a scene in the 50th anniversary that showed the Time Lords casually wielding weapons of mass destruction beyond anything we have on Earth. "Sir, there are Daleks in this populated star system with billions of life forms!" "Alright, destroy the entire star system.")
The Timeless Child could actually fix that. Time Lord society is actually built on the torture of and experimentation on a child. Regeneration is tested many times, which means that this child was repeatedly killed in order to recreate regeneration and give it to the rest of the Time Lords.
And then, in season 10, The Master (or Missy, at the time) gets a MASSIVE character arc. Even back in season 8, they showed a desire to reconnect as friends with The Doctor, and by season 10 was showing progress towards becoming more of an antihero, or possibly even a proper hero. However, in season 12, we get reintroduced to The Master, who is just evil again. Not a single ounce of that character arc remains. And we later find out that The Master destroyed Gallifrey. Again. After we just spent so many years working towards getting it back. All because The Master learned about the Timeless Child.
Anyways. It could be rationalized that The Master learned about the Timeless Child, and because of the newly rekindled friendship with The Doctor, they became insanely angry at the Time Lords for what they did, and destroyed them, and in the process of this genocide they slid back into being evil to cope with it. But that's not what they did, or if that was the intention they didn't really do a good job with it.
However. I argue that it would be better overall if The Master was the Timeless Child. Learning what the Time Lords did to them would have broken the Master, causing this singular burst of unbridled rage, which would end up destroying Gallifrey (or not, because I still don't like that Gallifrey was destroyed so soon after it was restored). Then, when The Master meets back up with The Doctor, they can be conflicted, because The Doctor is still their friend, but is also a Time Lord, with that aspect that was tortured out of The Master. So, The Master is constantly in an internal battle between fighting The Doctor or not. There's also this anger about how the fight for extra regenerations that The Master was fighting in the classic series was essentially just a waste of time. They always had more regenerations. They have this power in an unlimited capacity now, and that could go to their head, driving them insane with power, also explaining the backslide into villainy
It would also spark an internal battle within The Doctor, as they would now be aware of what regeneration truly cost, so the next time they regenerate they could reflect on what it would mean. Do they have the right to use this gift, knowing it was stolen from their best friend? Should they just let themself die? Or do they regenerate, and continue to pay it forward? Which is even better since The Master is who killed that version of The Doctor.
Again, I like this idea, I just wish it didn't undo a lot of work that the previous 15 years (at the time) had been building up. It just ignored a lot of it, when it could just as easily have enhanced a lot of it.
