Do you think Susan Foreman is dead or still alive somewhere?
Well, this is Doctor Who we're talking about. Susan is alive in lots of places on lots of timelines, and dead for more. And she is (we assume) a Time Lord and is (probably) the child of the Doctor's child, so that would add to the times and places she's alive in.
Or, she's alive when a Doctor Who Showrunner needs her to tell a story.
I thought she was Gallifreyan but not a Time Lord.
Given that she came and went from Doctor Who long before either Time Lords or Gallifrey were part of Doctor Who continuity, and that her only canonical post 1965 TV appearance, in The Five Doctors, doesn't address any of this, I'd say she's whatever a showrunner would need her to be.
I love it when Neil is canon agnostic.
Doctor Who in particular is a show that behooves this kind of thinking. Everything is true within its own context (within the world of the current showrunner), and so many different realities are true at the same time.
Or as the Doctor stated once,
"Sherlock Holmes solved the case before I could, as I recall." "Sherlock Holmes is a fictional character," Trix pointed out. The Doctor grinned. "My dear, one of the things you'll learn is that it's all real. Every word of every novel is real, every frame of every movie, every panel of every comic strip." "But that's just not possible. I mean some books contradict other ones and–" The Doctor was ignoring her. –Lance Parkin, The Gallifrey Chronicles
And as Russell T Davies said not too long ago,
All Doctors exist. All stories are true.