take it either semi auto only or pistols only, limited ammo. can zombies use grenades akin to in the original? remember you would have a bulk force would end up hiding in a room like they always did..
different game mode start. everyone speread out before team selection out of view of any other players. package given to each playe with team colours of one form or the other. fight to get to your own spawn. die before making it there and you are downed till revived. make it to your spawn and you from then on play normally, first team to have all their team returned wins.
and then just game maps from old good games... maby some split level goodness(be it under ground tunnels, or rooftop styled walkways linking buildings, or tree houses.... etc)