Maybe I will get scolded but why not focus on an era way before Geralt? When the human race just created the Witchers and they had all the work in the world? Potential not only for RGP but also CO-OP or even MMO style.
I agree that if CDPR decides to return to the Witcher Universe, this would be the best way to go. Avoid the existing cast of characters completely. Their story is done. Completely new cast, the early days of Witchers, etc. I think would work much more gracefully.