Dunno if this has been suggested, but how about different ground variations of Smooth Dirt?
We could have choices between grass, sand, and concrete. Would allow for more variation when building on them.
If changing the texture of the grass is too much trouble while in-game, perhaps separate flat maps for each texture type, since they're super cheap at 10k apiece anyways.
This way you could choose from building in a massive grassy plain, endless desert, or expansive paved landscape for cities/urban development.