Eh, idk. The Witcher universe has a framework for an RPG, but the books have always been about Geralt and Co. You could do something different in that universe, but I think the story and the games have some very strong structure that comes from the player being Geralt and living his story.

Witchers are a rare dying breed. Trying to shove another new generic Witcher in would force you into writing a whole new series of stories, and backgrounds, just for that one character, which would make it get lost in the mix imho.