I've also implemented some spiky floor and ceiling runners.
I think it's important to have a selection of non-destroyable hazards like this to mix things up, allowing timing to be involved, especially if they are running across a terminal location while you are trying to hack it.
Speaking of terminals, the original idea was to have 4 terminals per room that have to be hacked to unlock the door (representative of the 4 numbers per room in Zillion). But I decided that this might be a bit tiresome, so I'm dropping it to 3. As I was looking through the maps, there was always one terminal that was easily redundant anyway - 3 is enough to ensure the player needs to explore the entire room before exiting.
The last thing I added are pods. These won't actually appear in the first level as they are visually out of place, and more fitting in the darker, more cave like areas. They can be stepped on (possible strategic advantage?) until burst, where they release a swarm of angry flies. I was planning to have small flies only, but I accidentally drew the first one way too big and liked it enough to keep it.