I've been musing over that idea of a sprawling sketchbox underworld suggested in a previous post. This is a bit different from the "megamodule" approach to megadungeons that I suggested a while back. Sure, I still see the usefulness, even the need, for themed sets of rooms at scattered locations, but a sketchbox underworld would show a little more attention to the area around the modular parts.
The idea is: a sketchbox underworld used to be a city, so if you build it on the fly, you use the same techniques you'd use to build a standard city on the fly. Such as these techniques here. You start with an "undervillage" or "underhamlet". You map that out with some of those techniques. You use the population dice map to place other former settlements on the underworld map. You figure out what the connecting passages are like (burrows, subterranean river, wet or dry canal, chasm, rough-hewn lairs.) There would have to be a way to determine which sections have collapsed, which have been altered or expanded, which have flooded, which have become overgrown with fungus or other environmental changes.
Since I started thinking about this, I went back to the population dice map and started to redo it; I'm trying to merge it with the quarter system and the random town table, not only to make it more useful as an all-in-one town creation tool, but also to adapt it to sketchbox underworld use. Maybe I can get to the point where a GM could run a whole campaign using that tool, the Quickie Dice Tool, and GM notes created during play. The only thing needing prep would be a handful of megamodules to drop in from time to time.