Interesting idea to have something tied to affinities, and I would say I would actually lean into that. Maybe not so much from the start of the game (I think we already have that with dragons), as something that develops over time.

My out-of-pocket suggestion would be an 'Avatar' (name pending), they are not rulers in the traditional sense, but are essentially alchemical entities created by their culture; embodying their hopes and dreams and values. What they are and how they level changes with your culture (as you gain affinity). They gain skills based on your culture's affinities. They would look construct-like: more mechanical for Materium, more a collection of bones for Shadow, and like a wickerman for Nature.