Beowulf 5e, from handiwork game could be use as a inspiration. You are a hero (like, its your class) and you travel from land to land to find monsters to slay. But, most named monsters have a specific Condition that makes them invulnerable until X. Either you must find their true name, craft a special item or potion, find an incantation, complete a ritual etc. After X is completed, the monster becomes defeatable.
The game uses 5e as a core system, is low/null-magic and use only one class, the Hero. Its made for 1 player and 1 DM, but I think it could work well with more than a single player. Many features of the class boost the retainers traveling with the Hero; retainers are only in the background and once they used thei one Trait, they are unuasable until the end of the scene.
I think a mix of AiME for its classes, social and travelling encounters and the magic list from the loremaste book + Beowulf would fit the witcher perfectly.