The backstory is that everybody in the world is teleported into the same place in the world (not perfectly the same place, but same region; nobody gets squished together). Then, events play out where millions of people are dying and fighting to live, and there are epidemics of disease and crime.
The game would play similar to an early GTA game or like dead static drive, and would have a third person perspective. Lots of people would die, it would start with a relatively pristine environment in a city or something, and then quickly go to shit because everybody is trapped in the same place.
I have a feeling you could incorporate some story too, to keep the player motivated and give them a reason not to just kill themselves at the start (apart from playing), and it could be cool if the player met characters who would interact in a non-hostile way.
This would be a very huge-scale project which would require a lot of advanced videogame AI to simulate survival situations and human reactions, and could definitely not be achieved by a single person or small team. Even in the beginning of the game, it would have to be optimized due to the large amount of NPCs on screen and in simulation.