I actually think New York could work really well.
"But wait!" I can hear you saying. "Surely it will be an irradiated pit?" Precisely.
New York gets bombed to absolute smithereens- large swathes of Manhattan Island are underwater, while the rest is literally a glowing crater, only not flooding thanks to the walls raised up by the explosions, and the wreckage of skyscrapers. But the pit is not empty- it is inhabited by utterly alien life forms. Mainly massive descendants of radioteophic fungi, roaches, sewer gators, rats, and others. All are completely and utterly mutated beyond what they once were, creating a totally alien ecosystem right here on earth. And, crawling out of the rubble, comes hundreds of ghouls. In the radiation filled zone, they find themselves in a veritable paradise.
time goes on. A ghoul city state has emerged, based around the old world values of Democracy, and rule of law... With a twist. It only applies to ghouls. Initially, human populations which survived in endless sewers, metros and vaults attempted to wage war on the ghouls. But they lost- badly. They were enslaved, made to do work for the ghouls, with children being indoctrinated and ghoulified.
this practice has spread beyond subterranean human farms, with slaves being captured from the outer boroughs. The Lyons BoS, which has devolved into a cult, but this time honoring the memory of Lyons himself as a messiah believing that they must spread His memory as well as the sacrifice and resurrection of the Wanderer to the whole world, has formed a reluctant deal with the ghoulies who provide massive quantities of steel from fallen sky scrapers (and the melted puddles thereof). Meanwhile, a charismatic warlord attempts to rally the Burroughs against the ghouls, and commit a mass genocide.