I had two different ideas over the past month or two for adventure locations. One has to do with the distance to the first megadungeon of a campaign. Generally, experienced GMs recommend having the major dungeon within about an hour's travel from the home base, so that beginning characters can make short excursions and get back to safety to resupply at the end of every day. Some people have toyed with more extreme interpretations of this, placing dungeons right in the center of town, or going the other way and having an underground city in the megadungeon where PCs can resupply or recuperate. There's also been a general interest in ruined cities, subterranean or otherwise, or unfriendly subterranean cities as a design pattern for a megadungeon.
And I thought: Why not all of these? A formerly great city that has been partially devastated and is a shell of its former glory, with most of the city in ruins and overrun with various factions: the bandit quarter, the necropolis, the urban jungle. Sprinkled throughout the city are access points to underworld locations. Pretty much the entire economy of the remaining city revolves around salvage and reclamation.
The other idea was about subterranean towers. I've kind of toyed with this before, making two- or three-level fortresses surrounded by a chasm. But I read something in the Dwarf Fortress development page about twenty or thirty-level subterranean towers, and it got me thinking about a hundred-level tower, with occasional bridges anchoring it to the sides of an enormous shaft that plunges deep within the earth.
And then I thought: why not combine both? A city, mostly in ruins, somehow as a consequence of an enormous excavation. Travelers visiting the city to try their hand at "mercenary archaeology". Locals split morally on how they view this. The most daring adventurers braving the tower in the depths, rumored to be pure evil and the cause of the city's past misfortune.