My roommate Danaher has brilliantly postulated that the Doctor's current companion, Donna Noble, is, in fact, the Master in another human form. Though she doesn't know it, of course.
It completely fits. We see a female hand pick up the Master's ring at the end of last season. Various characters have been telling Donna that she has an unusual fate, is incredibly "powerful" and full of "change." Plus she occasionally wears a huge ring that could easily be the Master's ring with a bit of costume gemstone on top of it.
Just wanted to put this out into the blogosphere, to plant Danaher's flag as the first to come up with this call.