How's this for a setting idea?
All the world is a forest. There are many races in the forest: The Bat People, The Invisible People, The Fierce People. At the edge of the world, the forest ends and the land of the dead begins; people go there when they die. But there is a peculiar race that seems to have come from beyond the land of the dead itself: The Termite People. They consume the forest, building many things from the residue of trees, and every day the world gets a little smaller.
Sound like a good concept?
For those who don't recognize it, it's the state of affairs from the viewpoint of one of the tribes in the Amazon River basin in John Boorman's The Emerald Forest. The Termite People are actually westerners -- Brazillians, and those that work for them. But if you divorce it from its context, the literal images of the names are pretty suggestive, and might spawn some good scenario ideas.
What if you stick with the bog-standard medievalesque D&D setting, but have an invasion by (literal) termite people who are (literally) tearing apart the world, piece by piece, making it smaller?