Alright, since no one else seems to have any more ideas, I'll post some of my own.
--Duel Arena mode. Up to X amount of players (most likely 4), can battle each other for survival. The entire mall serves as the arena. The players each control a different character, and will start practically right next to each other. Players can either fight it out right there "battle royale" style, or they can run off to find a weapon or a vehicle to use. Weapons that are only acquired from defeating a psychopath cannot be found (like Adam's chainsaws). Before the game, players can decide which weapons and skills will not be allowed for use in the game (like sniper rifles or disembowel).
--Custom made weapons. For this special in-game feature, there would need to be an area in the mall containing several tools like a blow-torch, welding torches, table saws, and other such tools. In this area a player can bring back pieces of weapons or other materials to this area where the player can combine them in different ways. In one example, a player could simply attach a hunting knife to the underside of the barrel of a rifle, and the player has made a bayonet for the rifle. Another example would be the player could take say, metal railroad spikes, and then attach them to one end of a lead pipe, making the spikes stick out in different directions. The player has now created a very effective mace-like weapon. Another idea would be to take a bowling ball, put cooking oil and gasoline in it, then attach a fuse. Now the player has created a powerful bomb of some sorts. One last example would be that the player could cut a lead pipe in half, then connect the two halves with a short chain, and the player has created a pair of nunchaku (nunchucks).
[Edited by Molotoch, 2/11/2007 10:34:45 AM]
I like the custom made weapons, I wouldnt mind adding a pitchfork to the end of my machine gun. Or a teddy bear to the end of my Uzi. >_>