3 already takes place beyond the scope of the books. The universe is open ended enough for some clever writers to come up with ways to expand the series. It could follow Ciri, it could follow one of the lesser known guilds, there could be a Witcher profession rebirth...
Personally, I don't want it to be Ciri but that is only because it is more difficult for me to get immersed when playing as a female. I do think her story could be very interesting because she is a bad ass and basically has powers to do whatever she wants. That said, she can do whatever she wants so the story could get Uber cliche.
If it is not Geralt, which they said it wouldn't be, I wouldn't mind Letho's story. But that wouldn't be entirely original. Pop's story wouldn't be so bad as that predates Geralt by a few hundred years. Could even be from when Pop's was out the trials and such.
However, my heart will always be with Geralt. Once I finish RDR2, I may go back and do Triss's story unless I go back to Fallen Order.
It could be a horrible mmorpg...
Edit: Even though witchers are primarily solitary on the Path, it would have been pretty cool to have some seamless co-op elements like CP2077 will have. Imagine a night on the town as Geralt, Dandelion, and Zoltan.
