They won't go back to the main witch storyline, CDPR has already stated that The Witcher 3 was the last entry on Geralt's story. They said they could go back and apparently they're going back to the world of the witcher but Geralt certainly won't be the playable character, he might not even be alive in the new game. so there's actually a lot they can explore since they're not following Geralt's arch, they can just create a story for anyone, trying ot follow the books or not.
I mean, they can still do a prequel to the fisrt game. Or remaster the first game cuz it's cringy AF, the second game is still quite playable and enjoyable but I couldn't bare play the first witcher game again.