So, I got some more ideas and stuff
Classes can combine their skills, much like the characters in Chrono Trigger, to create... combined... things. For example, an engineer can team up with a demolitions guy and make an explosive kamikaze droid that will automatically target and eliminate as many zombies as possible. Or, a demolitions guy teaming up with a sniper to snipe explosives right into the face of a boss.
To make physics are more prominent part, and since most PCs by the time this game gets made, will be able to process near life-like phyics... so, a demo guy can lay explosives under a building, blow it up, and crush tons of zombies and block their path with freakin buildings!
Flying objects will knock down, or kill, people. This includes flying zombies launched by explosives. Other people can help people get back up faster. There can even be an upgrade for the heaviest armor for a spring that will launch out of your chest or back to get you back up on your feet!
PUNCHING. All classes can punch. You can upgrade your punch in various ways, cyborgs can mod one or both of their fists into a rocket punch just like Mega Man in Mega Man 7.
Pyros can toss oil all over the ground and ignite it to make walls of fire
Toss objects at zombies, along with your fists and the butt of your gun and some melee weapons, makes fighting zombies without ammo a little less impossible, but still pretty difficult.
And some concept art (mspaint FTW). This is just a very tiny portion of what the map would actually be, like, 0.1% of the entire map
http://i640.photobucket.com/albums/uu12 ... oncept.jpg
The black boxes are buildings, the yellow boxes are streets, the red boxes are alleys. The small blue squares are players, the small yellow boxes are player snipers. The red stuff on the streets and alleys are the zombies. There are so many of them, the snipers could easily no scope by randomly pointing at the swarm and clicking their mouse button with their eyes closed (which wouldn't be very effective though, because the sniper's job is to pick off strong targets and not generic zombies that will die easily to Gatling gun spray).
As you can see, a few people went into one of the alleys to grab some stuff that might be in there while the main force holds the waves at the exit of the alley so those guys can get out, but they don't have much time until they get flanked by the horde coming from the west. Snipers on tall buildings gives me another idea... ZIPLINES. Snipers can bring some tools with them to easily traverse roof tops like batman. The lines would snap if someone tried to use them with anything other than the lightest armor possible. You could also just jump, but your legs might not agree with that idea.
I'm sure modern PCs in the near future will be more than able to handle thousands of high poly models and near life like physics on huge booty worlds. Expononential growth FTW? I wouldn't be surprised if we had 32 core processors, 16 core GPUs and 512gb of RAM by 2015