It is is possibbble that she is anyone that the Doctor knows, according to RTD. However Julie Gardener is convinced that she is the Doctor's mother.
Although it is worthy of note here that the Doctor's mother is said to be human in the TV movie, which makes that possibility unlikely. However, it is unlikely we will ever know for sure in-universe who she is.
- Davies has said he intentionally left her identity a mystery so that future writers can expand upon it if they so choose. So far no one has followed up on it, but we can never predict what future writers might do. Although Gardner has expressed an opinion (and RTD I believe has also suggested his mother) it's what's on screen that counts.