Here's one thing I've wondered. If the Doctor could sense that there are no other Time Lords after the Time War, then, Susan must either be dead or not a direct relation to the Doctor. Not her real grandfather, just a grandfatherly figure she called Grandfather. Last we saw, if we don't count Legacy Of The Daleks, she was living on circa 2150 AD Earth. Well, technically, the last we saw her was on Gallifrey in the Death Zone. But, supposedly, everyone was returned to their proper time and place at the end of The Five Doctors.
However, wouldn't it be a great way to bring back another companion for a one shot story? I believe Carol Ann Ford is still alive. Susan could well be the last of the Time Lords in that event.