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I don't see where they'd fit into the game at all, unless you did something like you die in multiplayer and become a zombie, and to come back to life(get your gun back) you need to eat an opponent's brain(punch them to death with a different animation). But not like invasion mode with zombies.
I could only get behind zombies if some nutcase out there actually believes that zombies really exist and we really are on the verge of a zombie apocalypse. Most everything in the Conduit games so far has been based on a genuine conspiracy theory, so this would need to be too.