The Witcher: Project Sirius, which is being developed by The Molasses Flood, will include co-op, a non-linear story, and character classes for players to pick from.
This is according to a job listing for a narrative director at The Molasses Flood, which mentions the story elements, while another advertisement for a senior multiplayer designer tells us there will be "various game modes, player progression, and social systems." There are also hints to a class system that could even allow players to pick from different Witcher schools.
The Witcher: Sirius certainly seems a departure from the formula set by CD Projekt Red. However, with the franchise moving away from the story of Geralt, we can only expect future Witcher games to be trying out a few new things.
As well as The Witcher: Sirius, there are two other games set in the universe. One is Polaris, the next entry in the series which will be the next game released. Then, there's the remake of the first Witcher game, which is set to release after Polaris.