CDPR has never done multiplayer in a way that makes it reasonable to implement into an RPG, much less otherwise. I would first like to figure out what kind of RPG they're going to make this. Obviously it's open world, but is it going to be top-down, Third Person, First Person, etc.? What I most certainly do NOT want is another fable co-op that was horribly implemented for split screen and ends up with everyone mashing the corners of the screen and hardly being able to see yourself. With an open world RPG (unless it's rendered on external servers) You can't have two players in separate parts of the world, that's sacrificing a large amount of the computing power of the system. If there's very large cells, then it's probably going to end badly. If the cells are small, and mostly interior, then it would probably work as long as the players are confined to one cell.
ANYWAY, the list of exceptions and variables goes on. So first worry about how the game plays, then worry about multiplayer. Think about what's best for the gameplay of yourself, then figure out if it would work with multiple people.