Does it have to be in the Witcher Universe? Having a set universe with set rules is probably useful in this context, since you have sort of an idea of how to build your arsenal, but having a universe [i]similar[/i] to The Witcher and building our own new creatures and rules around the characters and their abilities gives us a lot more plots and creative freedom, without having to study up on the lore to know how to play and how to use creatures. That's just me, though.