As many would agree, the game has turned into large open world free for all. Most players shoot on site, with most of the reward coming from looting the other players dead body, other than trying to team up and work together for greater rewards. My suggestion is to have more difficult PvE objectives on the map. Instead of a barracks that spawns 2-6 zombies, have a place with high end loot that is near impossible to access alone. This could involve new kinds of enemies, hordes of regular zombies or other things. Crashed helicopters could have stronger enemies guarding them, or some sort of noxious gas that would make you sick, which would require medical attention similar to a blood transfusion. Possibly switches that require more than one person to activate, mabey in order to gain access to an underground bunker, using key cards that are only dropped on the other side of the map by zombies at a military base, for example. I personally have noticed an increase in the necessity to team up with other players on the 0.72 Namalsk update when BR and ERV are enabled. These things can make it quite difficult to survive on your own due to the increase in damage you will be taking.