On the flip side, I literally wouldn’t buy a Witcher 4 which was either a recent-past prequel or where Geralt was more than like 10% time played. Geralt’s story is over. Revisiting early bits of it is the exact same kind of wanking that people who want to endlessly go back to TOS/Kirk with Star Trek, or who are obsessed with an imaginary “further adventures of Luke” for Star Wars, are into (albeit less horrible lol). Kill me now honestly if that’s what they’re going to do.
I think the most likely cases for TW4 are Ciri’s adventures (I think this is somewhat unlikely because the world-shifting would involve so many assets), which would probably involve a certain amount of Geralt, or something set further in the past or future of that setting, and probably letting you play a Witcher of your own creation - or possibly even a choice of Witcher or a Mage.