Alright, since I'm 80% sure Doctor Who starts tomorrow, I'm locking in my crazy guesses on who Mrs. Flood is for the season. This is based off of nothing but vibes.
1. The White Gaurdian
My reasoning? Eldritch being in white. We've seen last season that Davies is pulling from Classic Who a lot, so why not someone pretty important to some of the otherworldly aspects of old Doctor Who?
2. Kronos
I'm still really bitter about the whole Sutekh and Ruby's mom thing tbh. I convinced myself that there was evidence that Kronos was Ruby's parent (they're a God, they don't really have a gender.) And since Sutekh called himself the God of Death, it would be funny if Davies brought in another being from Classic Who that was more neutral in their title of God of Death. And as you can see above, their form while on this plane of existence is white.
Also, it's almost 100% a thing that Flood's the one that took the gold tooth that housed The Master. In The Time Monster, Kronos was about to torture the Master for all eternity if the Doctor hadn't pleaded with them to show mercy. So maybe there's still some hard feelings?
And yes, both of these rely heavily on the fact that Flood was wearing all white for about thirty seconds in the Season 1 finale.
So Ruby's mom pointing at the Tardis (which had Sutekh wrapped around it) could mean two things.
She knew that they would Time Window and it was a warning to the Doctor and Ruby. This means that the mom could have future/past knowledge, meaning she could be someone we've met.
Ruby's mom sensed Sutekh and the point was to him to warn him to stay away from Ruby. I like this idea because that means she might be as powerful as he is if she can threaten him. Maybe since he's the god of Death, she's a god of Life? Like, equally as powerful or powerful enough to fight him.
Edit:
What if her mother's Kronos?
So I was trying to explain to my boyfriend, who has absolutely NO knowledge of Doctor Who might I add, some of my theories/evidence gathering. I pose it now to the theorists of DW Tumblr in the hopes you can use any of it.
(This is about the episode list that rtd said might be worth a watch before the finale btw)
Also after reading all of that the first thing he had to say was that Susan might be a version of Clara.