Speculation below the jump.
Ever since The God Complex, I’ve been thinking that the Doctor needs a companion with nothing to lose. The danger involved in traveling with him is really beginning to torment him, but he definitely needs someone to be his conscience and to be the fresh eyes that he can see the Universe with.
What if someone wound up with him who would otherwise surely be dead? Someone for whom every day of living is an extra day? Someone who would surely die if the traveling stopped?
I don’t know how you might set something like that up, but Moffat probably would. I have to wonder if Oswin’s set-up for the eventual arrival of Clara has something to do with this idea.