Cool, I like well-integrated settings like this. It seems a popular approach in some Nordic larps too, using their own rich traditions (they do have elves, but they are very unlike those of generic fantasy).

Slavic myth is lovely, we used a lot of it in Mordavia. I particularly like the spirits of the house and the land, and how they have their own domains and are not really good or bad but just elemental. We had a domovoi (played as helpful old house spirit full of premonitions, who could turn violent at those who endangered the happiness of the house) and leshii (a wild-spirit trickster with goat's legs who only talked backwards and could strike people mute), and worked in some key slavic folklore characters like Baba Yaga.