It's never been explicitly stated on screen what happened to Susan. However, it has been strongly implied that she died, probably in (or, at least, as a result of) the Time War. In "The Empty Child"/the Doctor Dances, Dr. Constantine says, "Before this war, I was a father and a grandfather; now, I'm neither, but I'm still a doctor." the Doctor replies, "I know the feeling."
I don't know, she was last seen travelling off in the Master's TARDIS, it would be nice to see her back, and Jenny too since she is also Timelord, the 10th Doctor technically being both her mother and father. There are Timelord allies out there for the Doctor, let's bring them back and see them working together again!
When was she "last seen travelling off in the Master's TARDIS"? So far as I know, she was last seen in "The Five Doctors".
The reference to Susan Taking the Master's TARDIS is from the Eighth Doctor Adventures novel Legacy of the Daleks by John Peel, which takes place after the events of "The Dalek Invasion of Earth". At the end of that novel, Susan comes into possession of the Master's TARDIS after he tries to capture her, and is once again able to roam time and space. This seems to be the last direct reference to Susan in the Doctor's timeline to date.
Lots of things have happened in novels without having been accepted or endorsed in the TV show. Certainly, the exchange between Dr. Constantine and the (9th) Doctor isn't a direct reference but it's a fairly strong implication and it postdates any Eighth Doctor Adventures. On that basis, I'd say Susan is probably dead but we can't rule out the possibility that she could be brought back, if the writers wanted. It seems pretty sure, anyway, that the Doctor believes she's dead.