Witcher is a licensed product, is not Dragon Age, it's like Batman Arkham series, means you can't do whatever you want actually, a new Witcher game will be about a new Witcher, that's for sure, it can't be about a mage or an elve, but just about a Witcher. It could be a prequel about a new legendary Witcher lived 400 years before Geralt, and he was a legendary Witcher in his time just like Geralt, or a sequel, maybe settled some century after the event of Witcher 3, when, somehow mages are started to create new Witchers again, to keep handle and protect people against new monsters/threats. But, a sequel with Ciri or a sequel/prequel about Vesemir or Eskel or Lambert, is really unlikely, those characters aren't " powerful" or special as Geralt, and if you make a new game you need a protagonist which is at least "strong" as the older one; Lambert, Eskel, Vesemir, are nowhere good and special as Geralt as protagonists.