I have been wondering if Susan Foreman and the Master are related. Ever since the Missy said she had a daughter it made me think that it's possible that the daughter is Susan's mother. In one of the Doctor Who Novels the Doctor hints that his son is Susan's father.
Now I'm not just basing this on just hints, I also think my theory is possible when you look at their hair color. Nearly every incarnation of the Master has black hair and light skin and Susan shares both of those features.