I disagree, the witcher world is very unique in the fantasy setting.

For starters indeed a lot of monsters are taken from traditional European folklore, but at the same time a lot of them are sparsely covered in common fantasy.  The world of the witcher has a lot of science fiction undertones with the alternate worlds and dimensions and a cool history of this melding between the human world and the magic world, rather than the typical fantasy trope of simply shared existence since the dawn of time. Lastly, like Game of Thrones, it represents a dark, gritty and vivid world rather than these happy gentle harmless worlds akin to Lord of the Rings. 

I hope the OP continues with this idea as there's a lot of potential