This is a great idea, lots of different scenarios you could play out.
Asteroid strike - worldwide reduction in food production for many turns, degradation of terrain quality for food near impact, but improved mining.
Plague killing population in cities across the world spreading city to city on a larger scale than the current event does, and with no option to spend cash to stop it.
Solar Flares/CME destroying all electric improvements on the planet.
Planetwide volcanism such as the Siberian traps volcanism of the distant past.
Alien invasion.
Zombie Apocalypse.
Some sort of pollution based event that changes the atmosphere.
I do think this should definitely be something the player chooses to play with, the idea of having thousands of turns laid to waste is going to turn some off.
Asteroid strike - worldwide reduction in food production for many turns, degradation of terrain quality for food near impact, but improved mining.
Plague killing population in cities across the world spreading city to city on a larger scale than the current event does, and with no option to spend cash to stop it.
Solar Flares/CME destroying all electric improvements on the planet.
Planetwide volcanism such as the Siberian traps volcanism of the distant past.
Alien invasion.
Zombie Apocalypse.
Some sort of pollution based event that changes the atmosphere.
I do think this should definitely be something the player chooses to play with, the idea of having thousands of turns laid to waste is going to turn some off.