I have some Tower Defense map themes that you could use, I think that a bone styled Death kinda themed map would be cool looking. Like Rib cages, skulls, built around that map, etc.
Another theme I have is an Outer Space theme, you could have dark sky, etc. Planets, and stars, the sun. Maybe some aliens, I know that Armagedon would make it dark, but you could still work with it since it has the notification.
Last theme I have for you is an kind of like the lobby, have a city themed map, buildings, and sky scrappers, Banks, etc. Streets could be the path the mobs walk on.
Another theme I have is an Outer Space theme, you could have dark sky, etc. Planets, and stars, the sun. Maybe some aliens, I know that Armagedon would make it dark, but you could still work with it since it has the notification.
Last theme I have for you is an kind of like the lobby, have a city themed map, buildings, and sky scrappers, Banks, etc. Streets could be the path the mobs walk on.