I was thinking about some kind of end game goal which wouldn't exactly be forced upon the player, but would be an option if one chooses to do so.
There would be a hidden underground military base placed randomly as a part of RWG or somewhere fixed in Navezgane. It would not be destructible by the player, somewhat like trader outposts. For the exact location of the base, you would need to find hints throughout the world, military camps and such.
As the base would be indestructible, it would require some kind of a key to open the massive sealed doors. For the key, I was imagining something like a keycard which could be randomly placed for the player to be found in harder biomes such as wasteland (the key would be placed near a randomly positioned corpse of a scientist/military/personnel member).
Upon unlocking the base doors and entering the facility, there could be some personnel/scientist/military zombies, and a scientist who has locked himself in a secure room with enough food rations to survive, but with no way to reboot the system and open the doors. He could instruct players via quests to repair stuff or find parts to free him.
Upon freeing him, he could then tell what happened like "some experiments went wrong", "they tried to find a cure but it's not possible" and offer a solution. There could be some kind of a beacon inside the base that could send a distress signal for pickup/nuclear strike on Navezgane. To get the beacon and all of the systems associated to work, it would be required from the player (quests given by the scientist) to repair the electrical grid and such with some high tier materials or recipes given by finding survivor NPCs (quest for finding them also given by the scientist) or finding specific books (scientist). Some materials for the repairs could be even specific to some regions/bandit camps when they are introduced to the game.
After fixing the base and rebooting the system, secondary base doors could open, thus making room for zombies to attack from more sides than just the original entry door (which would stay open to prevent player camping on horde nights).
The computer for the beacon itself could require a code that could be randomly generated and placed randomly somewhere in the nuclear fallout zone which would first require the player to craft or find a radiation suit and an item like oxygen tank or such so the player can't stay indefinitely inside the radiation zone but would require constant change of said tank.
After aquiring the code, and starting the beacon, a choice could be given to the player for a pickup or a nuclear bomb drop on Navezgane. There would be a timer on the screen in the base after the selection. The beacon itself would produce a sound that would attract zombies to the base (something like a horde night but with indefinite waves of zombies).
After the time runs out, a destination on the map is marked for pickup or... well... goodbye Navezgane, screen goes blurry, a shroom appears, screen fades to white.
End.
