I really, really, really hope CDPR will not make a sequel to Witcher 3, especially not with Geralt in lead.
Those characters had their adventures. Let Geralt, Ciri, Yennefer and all the others rest.
Maybe a prequel, way, way earlier than the story of the books ('short' after the Conjunction of the Spheres for example). Maybe where you create your own witcher, who is different from school to school.
But I don't want to see this franchise milked like many other franchises before. Now you love it and later when it gets out of hand most people dislike it, and I don't want this for Witcher.