Oddly enough I have had a similar idea percolating in my head for a couple years. The PCs are citizens of a dying population living in a giant ziggurat in the middle of a wasteland/desert filled with bones. There used to be many of these ziggurat cities, but contact with the others has been lost for generations. And those who have embarked to find the others, have never returned.
Every night, when the sun goes down, the bones stir, and a skeletal army led by spectres and wraiths form and assault the walls until sunrise, when they fall back into the sand.
The city is maintained through magical/mechanical sciences a long-time forgotten, and is slowly falling apart. And the population has been dwindling for decades. There are many parts of the huge city that have been empty for generations, and other things have moved in.
The PC's are members of an elite cadre of squads used to shore up breaks in the defenses during the attacks, relying on the advanced training techniques of their people.
But things are really getting bad... until a mysterious message/vision/phenomena occurs, and the desperate elders have no choice but to check it out, sending the best of the expendable defenders to follow this thin hope.
My idea would be to run adventures in and near the city for a few levels (maybe 5) and then have them sent out to investigate the unlikely chance of salvation (a 2nd city, a weapon, a portal out of the wastes, etc). After that, its all open to whatever I want, with the dying city always in the back of the characters mind. Kind of grimdark to start, with aspects of the fantastical to come later, as contrast.
I know this doesn't answer your post, but it reminded me so much of this concept that is bursting to get out, I had to share.