Snow height should be kept in a separate layer, probably the same one used for grass envelope now. The operations that modify terrain height (roads) should be extended to allow modifying only the upper layer - will be used for tracks in grass, snow, dirt. Ploughing/plowing falls into this category as well. Also the sand layer forming dunes in desert should be handled this way.
Physics and interactions are a different matter. Parts of it could be relatively easy, like the physics of wheels. But simulating it to the extent like in the video would be more complicated.
