The problem with a "Witcher" character concept is the inherent badassery required. Witcher badassery comes from their mutations, signs, potions, bombs, and sword coatings. Signs are basically spells, bombs could be gas poisons, and sword coatings exist in Alliance but are effectively worthless as they take a long time to apply and only work for a single strike, blocked or not. Witcher potions don't exist, as anyone can use potions, and Witcher mutations don't exist either.
There's more to Witchers than their combat prowess... the ability to track monsters and the knowledge of where they hunt, how they live, how best to take them out & such is also important... but none of that really translates to this game. The "Monster hunter" concept as a whole is certainly possible to do from a roleplay character perspective... but ultimately it's very difficult to discern yourself from everyone else who fights monsters during a weekend. I've been trying to build an undead-slayer type of character inspired by the Witcher series who carries a pair of swords (one iron, one silvered), but ultimately I feel like just another adventurer/fighter. Monster Slayer on a sword can kinda specialize you, and I'm certainly working my way toward getting one, but in general unless your fellow players are willing to sorta "play along" with your supposed expertise in hunting monsters... there's not much you can do.