Yotsuyu, the irredeemable monster

[Shuuchi]

Pull back on her wanton cruelty, make her older so that the worst parts of her childhood were BEFORE the fall of Doma, make her a believer in the empire and their idea of civilizing the 'savages' etc, who remembers the flaws of old Doma and you have an actual antagonist who opposes Hien's ideals and goals rather than just being a crazy sadist with a sad back story.

While I think your idea is FAR more interesting than what SE came up with, SB was focused around a cardboard box called Zenos.
He wanted someone as cruel and destructive as possible, not someone who actually cares and want to reform Doma.