Just write down and describe any possible game modes you might want to add!
Death Match: You spawn in an arena with all of the upgrades but no clones. Tougher and tougher enemies as well as terrain and traps enter the arena. After you defeat 200 enemies, you win! The whole goal is just to have fun the physics engine and just see how much chaos you can achieve. (I feel that this would go well with some of the other future implement ideas other people have.)
And maybe a "Create your own match" sort of thing. You could select what levels you wanted to play and when. You could also customize which enemies come in so you can either have a super-hard challenge or just have a bunch Mk:1 Swordsmen to completely obliterate.