As the big theme of the episode was coincidence, if that was something RTD wanted to continue the list would be:

• Some relation to the Ponds. Pond > River > Flood. Given the personality, maybe some future version of River that somehow managed to survive? I've seen others say Amy but the math doesn't work out from 1938 as it would make Mrs. Flood 110~ but who knows, timey wimey etc. Also, 'The one who waits' is what Amy did all her life.

• Some hidden relation to Ruby. The obvious one being her mother, but then the question becomes why did she decide to watch over Ruby like that from a distance? Also, Ruby was abandoned in Manchester and they lived there long enough for her to develop an accent before moving to London. Mrs. Flood was distinctly from London and perhaps just a coincidence again that they moved in close. Future Ruby/Ruby is a bi-regeneration of X character etc too

• Some relation to the Doctor. The Master is the obvious one after the tease we just had, and the Master isn't beyond breaking the fourth wall. The question becomes why would they aid her in joining the Doctor? The other usual suspects The Rani/Romana/Susan all would be coincidences but would we as the audience really care about it? I've seen someone say The Nun (the female incarnation of The Meddling Monk) which makes a little more sense as the episode is about meddling with the time stream.

Small aside: It's potentially a fake out that the woman we see abandoning Ruby is her mother. I've seen others say that Ruby could be her own mother even. It's certainly implied the character we see is her mother, but for all we know the baby from the 42nd Century and is found by Ruby during their adventures. Making a circle anyway while still keeping the mystique.

Knowing RTD and how he likes to tell stories, I wouldn't be surprised if 1. Mrs. Flood is just someone they meet in the past and it's a genuine coincidence. Nothing special, just someone who helps them out a bit. 2. Ruby's history is just of an ordinary foundling girl swept up in time. The mother who abandons her is Ruby after the baby is the only survivor to some future tragedy. Not special In the way fans think, but special all the same.