I think that's a bad idea. The universe of The Witcher has given almost everything, there is no sense on telling more stories of the same known world. Better concentrate on the Cyberpunk 2077 project.
That can`t be further from truth. Just by terms of explorations, the games cover a very small part of the known world, we haven`t seen Zerrikania, Kovir or Nilfgaard, and those seem to be the "nations" of the future. We don`t know whats beyond the sea and then we have this multi-verse stuff. The universe, on size alone, has much, much more to offer.
In fact, i would love an expansion to travel to Zerrikania, this games need a desert location. We don`t know whats beyond the Dragon Mountains in the north or the Blue Mountains to the east. The world of Witcher has still LOTS to give. Maybe we just need to trascend Geralt, as charming as he is.