If there would be a Witcher 4 game, then what sort of setting should it be in? With vesemir when he was younger?Another witcher entirely? What new features should the game have? How would you want it? Basically, just write down
what you want the Witcher 4 to be like.

I think will be great if they mix futuristic sic-fi with witcher universe. We all know that in witcher universe all "magic" explained as natural behavior of current world or some creatures from different worlds. So why not mix some dimension travelers (group of humans + non-humans(some sort of aliens) from tech. advanced universe) who escape to save some new technologies from *sshole who want to use this technology to dominate the world. For example 'white frost' could be explained as through-dimensional one sided energy dissipation that's deliver energy to *ssholes dimension. And when Ciri deactivated 'white frost' she triggered some actions.
Or by dimension we can say other planets from different galaxies bounded by some natural phenomena like randomly worm-holes (caused by advanced tech. civilization?). So conjunction of spheres will get new explanation.
WItcher 4 = advanced tech. + 'magic'.

That's actually a really good idea. I really want them to continue with the alternate dimensions thing.