Susan Foreman's mother

According to many accounts, there existed such a person as Susan Foreman's mother, although the details of her life and how she related to the Doctor depended on accounts.

Biography
On the day that she was giving birth, Lord President Rassilon sent guards to terminate the pregnancy due to it violating a decree which proclaimed only the Loom-Born could inherit the legacy of Rassilon. The First Doctor travelled to the past and saved Susan and Patience. (PROSE: Cold Fusion) According to another account, Susan's mother told her that she was the last child born before the Pythia's Curse. Her mother died during childbirth, but during her youth on Gallifrey, Susan's mother would speak to her in her head. (PROSE: Lungbarrow)

According to another account, she was a painter and survived childbirth. She taught Susan how to paint with watercolours before Susan left Gallifrey. (AUDIO: The Uncertain Shore)

According to yet another account, Susan's parents owned a concept shop on Gallifrey, and Susan lived there when she was an infant. However, this was only one of multiple coexisting pasts for Susan, alongside her being "Lady Larn" or the Other's granddaughter. (PROSE:  A Rough Guide)

Legacy
Upon seeing a picture of Susan Foreman, the Hermit said she took after her mother. (PROSE: The Three Paths)

While under the effects of a soporific beam created by the Soul Pirates, Susan believed she had "found Mummy" and was being taken to her. (PROSE: A Big Hand for the Doctor)

In an account where Barbara Wright was Susan's tutor, (PROSE: Doctor Who in an Exciting Adventure with the Daleks) rather than her teacher at Coal Hill School, (TV: An Unearthly Child) Susan told Barbara that she hadn't seen her mother and father in a long time. (PROSE: Doctor Who in an Exciting Adventure with the Daleks)

Parallel universe
In a parallel universe, Susan was the daughter of the Doctor's daughter. (AUDIO: Auld Mortality)

Behind the scenes
In a email reprinted to Benjamin Cook reprinted in Doctor Who: The Writer's Tale - The Final Chapter, Russell T Davies, choosing to deliberately leave the identity of the Woman ambiguous, anticipates that fans will believe her to be, amongst other possibilities, Susan's mother, "but of course it's meant to be the Doctor's mother."