I don't fully believe she is going to be a returning character from the classic series. I feel like she will be a new recurring character throughout S14 and possibly gets travel in the Tardis for an adventure, likely The Doctor comes and takes her away between the scene where she first sees the Tardis dematerialise and when he comes back to see if he's fixed Ruby's timeline (Possibly explaining how she is suddenly so friendly to The Doctor when he rushes back into Ruby's house, and why how she knows what a Tardis is at the end).
I would love to hear everyone else's ideas and theories on her, we have 5 months to speculate anyways.
