Everything I see is pointing towards an Origin Story that the show has never explored, just alluded to. And I hope it remains just that. But this may be what Moffit is aiming for in this arc.

We saw in "Listen" the good (?) Doctor's humble childhood. "Fear can make you kind."

We know that the Doctor and the Master have a long history - back to childhood. Is this the first time the Master was Missy? (Best incarnation of the Master ever, BTW)

The Doctor has grandchildren, or at least that's what was presented in the very beginning, so there is always that to explore in an origin story.

We'll see. I enjoy this regeneration, he's a smart-ass.

PS: Still watch it on cable, when it airs. Or if I miss it, they typically show the previous episode before the new one.