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A program with image recognition picks coordinates, semi randomly, finds information about that location, uses databases and search engines and maps to recreate it. Uses a second program to modify buildings and textures and terrain to be 'post apocalyptic', run down and distressed from time, bombings, zombie attacks, war, etc.
It does this over and over, creating as many points a day possible and then expanding to 50 mile radii. At release, players are distributed among these points as the program continues it's work. Instead of having the entire world complete at launch, only 1% will be done, but the program will keep working, recreating the world as people move and travel, someday eventually being complete.
On top of this creation engine is the ability for it to change through player destruction, building, etc. A player can venture out, literally, to be alone or stick around others.
Random generation would fill in scavenging supplies, enemies present, npc settlements and possible missions. Other points in the world, unpopulated by players at first, will be chosen as hives for whatever adversary caused or is a result of the apocalypse. These colonies grow, expand, and advance in power, while consuming resources as they go.
Players fight for control, freedom, survival, or sick satisfaction. They can choose to let certain hives wipe out their adversaries, or work together to beat it back. There's the possibility of humanity becoming extinct. In the case of a zombie apocalypse, a player can only rebind himself a few different times, having to rebind any time a hive takes over their current bind settlement/point. When that's used up, the next time they die is permanent; they can play as a weak/med zombie, until a certain time expires and the server resets, zombies win, or players free the last bind point the player was bound to.
Some character progression may be retained between 'seasons', and the patterns and tactics used by zombies/hive could change each time, sometimes with direct developer/GM direction, and partially based on what player zombies do. Deeds, accomplishments, wealth, etc will be enshrined in the history books for each season.
A season, where permadeath and some sort of win condition become plausible, would happen only with certain triggers, even if it took years to get to that point. Then there would be 90 day limit on how much longer the conflict, and world as they know it, would last. The goal would be 1 year between server resets, possibly restricting player starting areas based on overall population to a certain continent/country, and having each one free or doomed by the end, then moving to another one for the next season. Excluding antartica, even doing it this way you have several years of varying world to explore.