What it says on the tin. What kind of POD would it take, and how far back would it have to be, for Rome to be in a social position to allow a female ruler? Would this POD butterfly away Greco-Roman civilization in the first place, or could it be modified so that it was slightly less patriarchal (not in a "perfectly fine" sort of way, but in a Wu Zetian or Hatshepsut sort of way)