It's pretty simple, and shouldn't be too hard to implement given there's co-op enemy AI built into the game. Basically it is what it is...You and a couple other people choose a class and attempt to make a stand against increasing waves of stronger zombies. This would really promote a healthy class mix since you will likely always be next to each other. You could edit down existing maps(Sinner's Gulch graveyard), or create mode specific maps(old church). Hey, zombies sell, and they're fun to rekill.

This..Would...be....so...

EPIC !

Night maps + Cowboy Zombies..OMG !

a little bit too linear?

But there already IS a survival game mode! Oh an BTW, Zombies dont sell me, ALIENS do! (Or the 1800s)

i love zombies , in the cod waw mods

Why is everyone so obssessed with zombies? To fit the game mode it should be something along the lines of Indian or bandit or hillbillies. Think the magnificent 7 stand off at the end of the film. The only problem you would run into there is offending a parts of the general populous. Zombies or aliens just don't fit the game or the atmosphere it is built on.