As the title says, I think it would be an interesting medium-term/long-term feature to develop textures that change when driving in different environments. Essentially, you would have a partly-transparent texture overlay for a given vehicle that has mud or dust pre-drawn on it, and as the vehicle drives in muddy or dusty environments the game engine could increase the opacity of this texture so that it looks like the car is gradually getting dirtier as it drives. A lot of games use this method (eg. the Dirt series, GTA V) and IMO this is the most straightforward way to allow dust, mud, or snow to gradually build up on vehicles. One issue could be having to deal with storing multiple variants of this texture (for dust, mud, and snow) in memory for a specific vehicle, or storing these variants for multiple vehicles in a session, but I think the idea is interesting enough that it should be looked into at some point (unless the devs have already thought of something better).