The two of you seem to be forgetting one small fact. Nobody wants more Witchers, not even the current Witchers, they're a dying caste, so unless a second Conjunction of the Spheres happen there is no need for new Witchers. Even if a second Conjunction did happen, man has already conquered much of The Continent, so much so that monsters won't nearly be a huge problem as they once were.
That would make the idea of a prequel or game set in a different part of the world than the Northern Kingdoms make sense, then. The time and location of where TW3 leaves off arguably doesn't need new witchers, but different locations and/or periods of time could.
Unrelated to this topic: I'm watching some of the FO4 videos, and the discussions with Preston Garvey highlight one of the unique features of TW3 compared to many RPGs. Preston tells us that he isn't in a position to rebuild the Minutemen but we can. I haven't played the game or have much context, but this just seems like another power fantasy where the PC has to fix everything for everyone, and no one else is as capable as we are.
Things like this really stretch my suspension of disbelief and annoy me. Really? The guy who was a leader of the Minutemen is not in as good a position to rebuild them as a complete stranger is? Granted, I could be missing context and details, and maybe it does make sense if I would actually, you know, play the game. In which case ignore this. But if not, I'm tired of playing the one person in the world who can fix XY and Z.
That's one of my peeves with the Inquisitor's special snowflake status with the Anchor. And one of the refreshing things about TW3. Yes, our actions end up having significant impact on the world, both politically and in stopping the White Frost, but the central story is much more personalized and is about Geralt seeking and protecting Ciri, and helping her embrace her destiny. We are an important figure, but we're not the only person who can save the world (in fact, Ciri is better able to do so), and we're not the only person in a position to help (the battle at Kaer Morhen and the sorceresses' help in Skellige prove this).
It's nice to play a game where we turn out to be inconsequential despite our actions (DA2 though I enjoy the game) and we're not the World Savior (DAO and DAI), but we still have some part to play in the greater scheme of things.
Plus the optional questline with Radovid, which has lasting implications but is in no way necessary to advance the game. Another great feature.