I couldn't come up with a lot of layers besides the ones you already had, judging from the GDD I don't think there's a lot more to add.
The layers I thought of were:
13. Enviroment (trees, rocks, bushes, grass, etc)14. Particles (Details that are part of the enviroment but are usually in the air like little insects or stuff like that)15. neutral units ? (I'm not sure if there are going to be neutral units wandering around in the map so i just threw this out there really)16. Projectiles (Things like rockets that come out of rocket launchers, helicopters, tanks and stuff. Maybe if you plan to add bullets to the basic attacks too they could be in this layer)