You know, we've got mods based off of things like Myst and Portal, why not have one based off of The Witcher? Players could find, dotted throughout the landscape, ancient Witcher strongholds that have since been abandoned where you can find information on making the mutagens used in the Trial of the Grasses, allowing a player to transform themselves into a Witcher, as well as books detailing Witcher forging and fighting techniques so you can create and use Witcher steel and silver swords. It'd also add things like
kikimores,
giant centipedes, and
drowners to the world; things only a skilled Witcher can hope to take on and succeed against with any regularity. Of course, to offset this, players can also find books on Witcher alchemy, allowing them to make potions (that poison anyone not a Witcher due to their high toxicity) to enhance their combat abilities far beyond that of vanilla brewing.