Who is Mrs Frost?

The caption on this, delivered in perfect Hermione voice by a small child: He must have crashed it. Again.
Doctor Who 1.9 “Empire of Death”
So, Sutekh has been kind of tagging along with the Doctor, and now he’s using that to kill every planet the Doctor has ever visited.
(The Doctor claims that’s all of them, but that seems unlikely. Still, it’s a lot of planets).
He possesses Mel. But he can’t touch Ruby…why? Because he doesn’t know who her mother is. He’s a god, following god rules here.
This leads to everyone not feeling so good and turning into dust. Okay, without the not feeling so good part. It doesn’t work as horror, because we all know the Doctor will save everyone. It does work as drama.
And Ruby uses the secret as bait to lure Sutekh into the Time Vortex, where the Doctor somehow uses him to restore everyone. Then kills him…which the Doctor never likes doing. But he didn’t really have a choice.
Every dead world where the Doctor landed is alive again.
…every dead world? I do think somebody just opened the door a crack to restore Gallifrey, didn’t he. Wonder how many people noticed that.
Ruby Sunday finds her mother…an ordinary teenager who left her at the church to get her out of a dysfunctional family. Nobody special. Kind of the point.
She leaves to get to know her mother and possibly even find her father…the Doctor is alone again, although rumor has it she’s in at least some of the next season.
We still don’t know who or what Mrs. Flood is. She might be a Time Lord. She might be a god. What she says to Cherry Flood implies she’s Satan, although she’s also a living being because Sutekh kills her too…
I trawled the internet for theories and came up with:
1. Susan
2. Romana, based on the coat
3. The God of Stories (I like this one because of the fourth wall breaking)
4. The Master (Davies says no, but showrunners lie).
5. The Rani (A popular theory, but I think not)
Who has one to add?