Is Alex's Time Lord ancestor someone we already know from the series? Or are they an OC Time Lord? Cos if its the first one, I have a list of theories for who it could be
1. The Rani- The most iconic other Time Lord except from The Master. Mating with another species would certainly work with The Rani's geneticist MO, but she like The Master almost feels too obvious
2. Romana- This to me would have great poetic payoffs, as a former companion and close friend of The Doctor being Alex's ancestor would feel like a good story line.
3. The Master- The most iconic other Time Lord and has utter chaos potential but feels way too obvious and very out of sorts for The Master to have a child with a "mongrel human" as he calls us
4. The Monk- A outside the box idea, that would throw off 11 I think. Having an echo of an old enemy be part of his lover's bloodline would surprise him imo
5. The Corsair- Again for me, this has beautiful tragic poetic storytelling, The Doctor's other old Time Lord friend aside from The Master, and who 11 and Alex know is dead, I could almost see Alex getting the snake tattoo Corsair has if this is the case
6. The Valeyard- This would be a very bizarre twist since yes The Valeyard is canonically his own being seperate from The Doctor but is also an amalgamation of the darkness of The Doctor. But The Dream Lord was another Dark Doctor who cared more for Alex than anyone else. So could The Valeyard have ensured Alex's Time Lord DNA to then ensure his past alternative self ended up with her?
And finally, the one I most believe
7. Rassilon- It makes almost too much sense in my mind. The mad king of Galifrey, the crazy old power mad dictator who always wants to survive. Ensuring his legacy and potentially his replacement if he could find a way to ensure someone in his bloodline could regenerate. If Rassilon did this way before The Time War, I could even see it being shown that he was partially behind Demon's Run to force Alex to gain the ability to regenerate, only to siphon her regeneration energy
Yes, Alex's Time Lord ancestor IS someone we know from the series! Ooh, EXCELLENT theories! I can't say which one it is, but they're all awesome (major shoutout to Rassilon!) so you'll just have to wait and see!