What about an Occlusion Map which was essentially a distance away from other surfaces? This could be used as a shader for moss (lower distance values have more moss?) or as a wear and tear shader (more exposed edges might lose their color and shine).
Also I think a pretty nice one to have would be normals. It would be easy to put snow on mountains or water on top of objects. (Or maybe more moss underneath rocks!) The normal map could work as a primary direction, and then difference from that direction. Normals could be local to the center of the mesh, or to the world (both would be useful).
These are just a few ideas, I'd love to hear more from the community!
