Zombies: Arachnophobia. Thats right, the two biggest fears, zombies and spiders. Imagine seeing thier shadows skitter through the rooms. I feel theres lots of opportunity to make these things incredibly creepy, applying the usual zombie skin with that of a spiders looks epic in my head.
They leave cob webs, which one must knife through as these can block doorways.
Levels 1-4: few spiders, smallish in size-about the size of a soldier squatting down.
Levels 5-9: start getting larger, more few. Couple of jumping ones-these will shit you up.
Levels 10-15: mother spiders start showing up. These stay outside, and start with one per level. These are very big, but can shoot eggs that hatch more spiders and can also shoot acid on to the floors that act a bit like lava on Town.
Obviously as you can imagine as the levels get higher, picture all those spiders running in across the wall and ceiling, and them popping out the ground.
Also throwing a grenade at zombies sometimes creates crawlers, with spiders what you get is the spider body being dragged along by 2 front legs. NIGHTMARE.
This could be the ultimate game changer for Zombies.
Just a suggestion