Doctor Who - but how does Rassilon, founder of Time Lord civilization, fit into this"
The Doctor is the Timeless Child, a mysterious being found at the foot of a foreshadowed fairly well by classic Doctor Who, though it sits uncomfortably with the post-2005 relaunched series.
There is one curious omission, however: Rassilon. Generally regarded as the founder of Time Lord civilization, Rassilon was an explorer who longed to see more of the universe than any man could in one lifetime. According to classic Doctor Who, and countless previous tie-ins, under Rassilon's reign the Gallifreyans discovered the secret of regeneration and became Lords of Time. Rassilon himself is immortal, although he believed no other Time Lords were worthy of this, and left the Game of Rassilon behind as a trap for any successor who would seek immortality. The Time Lords brought Rassilon back to lead them in the Time War, and he was opposed by David Tennant's Tenth Doctor in "The End of Time."
Rassilon is apparently entirely absent in "The Timeless Children," which seems odd given that Chibnall is an old-school Doctor Who fan. It is important to note, however, that "The Timeless Children" confirms Gallifreyan history has been deliberately obscured; that leaves a lot of wriggle-room. The most likely solution is that Tecteun, the scientist who discovered the Timeless Child and unlocked the secret of resurrection, was in fact Rassilon. Both have been portrayed as explorers who sought immortality; Tecteun gained the power of unlimited regenerations but chose to limit other Time Lords, just as Rassilon believed he alone should be functionally immortal. It's true that Tecteun was female while Rassilon was male, but that is hardly an issue given that regeneration allows a Time Lord to change biological sex.
This is a simple retcon, and it fits rather well. If this is the case, then during the Time War the Doctor found himself going up against the one being who truly understood what he was capable of. Little wonder Rassilon was so shocked and surprised in "The Day of the Doctor," when he realized how many incarnations of the Doctor were in play. It is ironic that Rassilon, the explorer who sought immortality to explore the cosmos, ultimately led Gallifrey to a pocket dimension where his people lived on in isolation, unable to return.