Since everytime we play 7Days Co-op it kind of ends when there is no more real threat coming from the Horde nights, I suggest adding an endgame biom. It could be a radiated area at the corner of the map, which you can only enter once you collected (or crafted after reading a specific book series) the hazard armor. (and then still only for a limited amount of time). In this area there are strong zombies (only radiated ones and maybe some unique ones like radiated dogs and birds) and also good loot. In the middle of the radiated area could be something like a nuclear plant building with especially good loot. That way you would still have a mission / purpose once you mastered horde nights (of course they get tougher, but after a certain point you will just need some repairs and ammo crafting to overcome a horde night).