I usual problem I see with most zombie games is the variance in zombie height. There isn't any. If a horde of zombies is approaching you and they happen to all be 5 foot 9, it's not hard to pop them all in the head. However if they scaled the models down slightly and up slightly. You could have a range of 5'2" up to 6'4" and have one hell of a time trying to kill a horde. Make some zombies hunched, heck if its alright with the devs they could add children zombies like "No more room in Hell". It's a pain having to look down to shoot a small zombie when you've just been shooting tall ones the whole time.