I would play pretty much anything, but be warned that I don't know anything about the Witcher or Warhammer universes
I can pretty safely guarantee that that won't matter, but players would have to play by the rules of whatever universe they're in, that's a given no matter what universe. Magic =/= god powers.
For those interested, Witcher would cover the Northern Realms and, like, the top half of Nilfgaard, for balance reasons. Nilfgaard would probably be managed by AI, too, as well as the Hierarch of the Eternal Fire (kinda like a pope), but both's vassals would be playable in theory, like Cintra, Toussaint, Nazair, etc. for Nilfgaard and the Novigradian Patricians (MAYBE) for the Hierarch. I'm also fine with the Hierarch being playable, but there'd have to be some rule about succession if that becomes relevant.
As for Warhammer, I'm thinking the Empire, the dwarf holdings, the wood elves, Bretonnia, Kislev, Estalia, Tilea, the Border Princes, and Orc, Chaos, and Chaos Dwarf invasions, either controlled by players, or AI if not. Ulthuan maybe, if I manage to do all the other stuff, but that's already a lot of work. Noscrea would come before Ulthuan in priorities, but I'm not even sure how Noscrea would be done, since there's minimal unity there. The Empire regions would each be playable, I guess, with each being able to fight/plot for control of the empire, and the emperor would have some unlocked advantages when the empire is unified, I guess. Warhammer is probably not going to happen, realistically, unless there's a lot of support for it.
I'm in for basically anything but GoT. I may also help a bit with mechanics and writing if the host would want me to.
GoT was my last pick, personally, because I have little interest in a lot of the houses, and the liege-vassal system would be a huge pain in the ass.
Added a poll, if you're willing to help (I understand you might not want to depending on whatever theme it is), fill that out.