Why don't you include natural hazards? Not just drops, stuff like turrets and evil bad guys or something.
There will be turrets in the next world !
Cool. I reckon you could include spikes or something too if you wanted. And if you ever make a scifi themed map (I haven't played much, so whether you already have one, I don't know), a good idea would be to have a laser firing vertically down the middle of the map, on a timed interval. You could have a charging animation too, if you wish. For a better idea, search strike force hero's 2 and look at the tutorial.