The idea of a super-evil female villain is appealing, but I would prefer by far a grey-shaded complex figure. Not because I am obsessed by grey characters like someone (in fantasy/sci-fi stories "white" and "black" characters works perfectly as archetype of human beings, so they are permitted), but since Radovid (at least in T3) and the Whild Hunt belong to this category and in my opinion Olgierd fails as grey-shaded figure (since you are basically forced to help him because Gaunter is too evil, despite I didn't like him at all), I would prefer a complex villain with romance dynamics and real big compromises involved. Maybe a noble beautiful woman who is a vampire, in that case Geralt's issue about sentient monsters would be examined in depth, with a romance dynamics which is always engaging and some politics involved too. A sort of Iris figure (the best character of Hos imo) but more central in the story.