Oswin as a time lord or time lady is quite plausible. You know at first I thought she was Susan and just didn't know she was Susan (the Doctor has had amnesia before)... but ehhh I think she could just be another time lord/lady.

The Doctor might have been the last time lord... but um, the whole time travel thing, Rose's universe of current residence, Susan Foreman (death hinted at not confirmed), Time Lady Romanadvoratrelundar (Romana) is MIA, River, unknown race 1 or more time lords tinkered with similar to the Minyans (or a Minyan that survived), , Spoiler Alert: Boe told the Doctor he wasn't alone as he died, Master once used a Chameleon Arch (and I'm 100% confident the Master will resurface once again, hell Clara could bet he Master!), another time lord could have, she could be Lucy Saxon, etc.