So now this idea is mainly about making a grenade that dispenses (not sure if I spelled it right) deadly smoke, which (ofcourse) dissappeares by time. The better (higher level ) is the nade the more, the smoke stays. Now there will be 2 types of grenades: 1 smoke that WILL NOT do any harm if you have a gas mask on. And second one is the one that will do harm to the body itself. So if you have a gas mask on 1st grenade will not work on you but 2nd will (but if you don't have the gas mask on, 2nd will kill you a LOT more faster). Now the smoke will also work like cover. So if you have guys with gas masks on they can use this grenade like a simple smoke nade.
And ofcourse it will work on zombies..... but: not on elite scientists and gas mask elites (I hope that con really makes elites with masks) and some elites will even have high (very high) resistance to melee and firearms but gas nades will take em out easily.
Now this is just an idea guys tell me what you think :)