easily a zombie survival one would be great.
A countdown(in turns) until zombies start heading towards/attacking your base.
You need to establish your town (base) including defenses, roam around and pick up any other survivors and weapons/items you can find to help you and then move on to the next town (level)
OR
Make it like a normal map but once a zombie, or group of zombies, sees you they will keep moving towards you until they can attack you, AKA eat your brains. Zombies could literally be everywhere and could destroy other towns, upon destroying towns the units within will also turn into zombies and you basically have to save people to help you kill them off or get totally overrun when the entire place (except for you) are zombies.