I think it's more about making the tools for character creation more than anything and the more complex they are the more variations of faces you will have.
However that's not guaranteed to work better than what The Witcher games, if I'm not mistaken many of the faces were based on real people and you get more belivable characters that way instead of randomly generated characters who have no ...character
Not "many" based on real people. Some where. I think they need to do something to improve on TW2 (and, like you, I don't think that this is just about the female characters, there's a lot of repetition among the males too, especially in the faces).
I don't think they can probably use a lot of different models, nor do I think it needs it. Multiple models have a performance impact. But there can be a lot done with a single mesh to make radical changes in appearance and body-shape.