This thought crossed my mind earlier today too. To help players cooperate amongst each other then just kill each other: make some high-end areas of the game accessible unless you have 2 or more survivors helping each other access it. For example, helping get over high walls, getting on top of roofs and large vehicles, lift/push large objects out of blocked paths etc. Even to the point where you can have high security military installations that need dual user access, like using two ID cards scanned at once to access an armory.