have multiple weapon skins and have it so there are no weapon spawn points, just the weapon on a table or a curb. also start off each campaign with either on of each kind of tier on weapons (smg,shotgun)and give a melee weapon or two or start with two of each kind of tier one weapons. also instead of finding inexplicable weapon stalk piles find only one or two weapons per map, for example...
lets say that two survivors have guns and the other two only have pistols (or maybe a melee weapon or two) and they come across a dead farmer with a shot gun, vowella! this type of way of finding weapons could be applied to any other weapon, military locations and police buildings could have armouries of tier two weapons (assault rifle, auto shotgun, hunting rifle) and you could just find weapons randomly throughout the world.