There are many competing theories, I suspect that she may be a future regeneration of Jenny from the Doctor's daughter, a future regeneration of the doctor is possible but a bit old hat after all the Valeyard was an entity created between the Doctor's "twelfth and final incarnations".
I had honestly not thought off that, sounds like it would be an interesting twist though
I'd thought you'd at least tell us what the scoop on Riversong is? With all your bigshot BBC contacts
If I told ya I'd have to kill ya Or is that they would kill me
Ve have vays of making you talk englanderr