she may have fought and died in the time war
But I thought that the Doctor left her on Earth with David Campbell?
Well, it's never been categorically confirmed that Susan is/was/will be a Time Lord/Lady. "The Five Doctors" confirmed that she's Gallifreyan, but we don't know if she actually attended or at least finished the Academy. It's fairly clear that not everyone from Gallifrey is a Time Lord.
However, the Doctor has stated that his family is dead. It's possible she died in Earth's future and the Doctor's ability to see fixed points in time gives him the knowledge that he'll never see her before her death.
There have always appeared to be rules in place in the original series regarding the Doctor's encounters with others of his race (other than his own previous selves). The televised battles with the Master were always in sequence. Likewise, his trips to Gallifrey happened in the same order for him as his fellow Time Lords. It's possible the amount of time that passed for both wasn't the same, however. Borusa regenerated 3 times, while the Doctor only regenerated once (fourth to fifth) and one of Borusa's incarnations appeared quite old ("Arc of Infinity").
We don't know what killed everyone in the Time War: and don't forget, Gallifrey had the Time Scoop, they might have used it to bring every Time Lord into the War. Plus (spoilers ahead) in possibly-apocryphal novel Legacy of teh Daleks (EDA), she got a TARDIS of her own and left Earth.
It;s also debatable whether the Doctor truly is the Last of the Timelords. This belief stems from two reasons:
- Gallifrey was destroyed. The Doctor presumed all the Timelords were there at the time.
- the Doctor claims to be able to sense other Timelords.
These issues are in doubt. The Doctor of course has no way of being certain that every Timelord was involved in the war, and the series has proven numerous times that his ability to sense other Timelords cannot be taken as absolute proof of fact. In the Doctor's Wife, the Doctor really believed there could be other Timelords out there - but he wouldn't have been able to sense those either, so we have evidence to prove that the Doctor himself knows that he cannot trust his senses alone.
Not all Gallifreyans were Timelords, and neither do all Gallifreyans regenerate. Since it was never actually mentioned onscreen whether Susan even had the ability to regenerate in the first place (which is likely since she left the series so that she could 'live her life', before the concept of regeneration was introduced), so she could have died naturally many years in the past. Such an event would have been traumatic for the Doctor so it is understandable when he appears to refuse to rake up the past.
Since Susan had a human boyfriend/husband then it can be assumed that the Gallifreyan DNA of Susan would have been watered down over the generations, culminating in the Doctor having descendants who would have been entirely human.