I'm kind of a sci fi junkie, so my suggestion would be to make them exist in alternate dimensions/realities and be connected somehow through either a scientific experiment gone wrong or some mystical accident etc. Make several different levels where you have to survive so many waves, kill so many bosses, or whatever to advance. Maybe throw in a little storyline between each level, but just enough so players imaginations can run with it. Have levels with different settings, inside buildings, underground, forests, etc. Just my take on it though. And you don't need too much gore or whatever. yeah blood and violence is cool, but you want them to see the awesome graphics you produce, not just red. Just saying.