Every civilian and soldier turns into zombies.
Hear me out on this one...ok!
You would be able be able to turn this mode on and off. The tasks you are given must be carried out. For example: Rescue a bunch of stranded reapers, Obtain an armoured vehicle to escort them back to a safe base, Blow out zombies brains! Every zombie would come running at you. Elite soldiers would be bigger zombies and normal soldiers would be slightly more well equiped than civilians. It would permanently be night with the moon out!
What do you think?
It would be awesome!