Even as work proceeds on Dwimmermount, I found myself intrigued by another concept: a vast, ancient, and ruined city stretching miles in every direction, now populated by scavengers, tribes of humanoids, and leftover horrors from whatever cataclysm laid waste to the city in the first place. If it all sounds a bit like Gamma World, that's part of the appeal. It's never been surprising to me that Metamorphosis Alpha (set aboard a generation ship fallen into chaos) and Gamma World (set on a post-apocalyptic Earth) were among the earliest RPGs. After all, the very idea of exploring -- and looting -- the ruins of the past is only a slight modification of D&D's basic premise.
The main difficulty of using a vast ruined city as the basis for a "dungeon" is that it's not constrained geographically. That is, players can very easily move around the edges of the ruin to avoid any tricks or traps the referee might have placed in one area. Now, on one level, that's solid old school thinking there and ought to be rewarded. On the other hand, it's perhaps a bit too easy. Consequently, planning a dungeon of this sort takes some forethought. The simplest approach might simply be to use a ring-like structure, with the outer portions of the ruin corresponding to the topmost levels of a traditional dungeon, while the inner portions correspond to the deeper levels.
Even this approach raises issues, particularly once magic starts increasing player mobility. My general rule, when encountering seeming difficulties, is to embrace them as opportunities. Yes, players will eventually gain access to spells like fly and dimension door that might enable them to "skip" certain encounter areas. Rather than fret about that, why not give them something else to do with such spells? In my campaign setting, the Eld and the Thulians are the two primary sources of ancient ruins. Both were masters of magic, so it stands to reason that their once-great cities were built and maintained using magic, magic that lingers still after all these centuries.
So, I'm thinking my ruined city will have several floating islands associated with it -- former wizard towers and research facilities and other such arcane treasure troves. These will be magnets for mid to high-level adventurers who have access to magic and magical items that enable them to move around more quickly and easily. No, this won't prevent PCs from "upsetting the balance" of the surface ruins, but that's fine. Dungeons are made, after all, to be kicked over and looted by adventurers. The trick is to ensure that, no matter what, the process of kicking over and looting is challenging and that requires a referee who can think on his toes and respond appropriately. That's why I'll probably be a bit looser about the inhabitants of the inner ring of the ruins, so I can change them up as needed. I won't leave the area blank entirely -- that's just a recipe for disaster -- but their strength and numbers can be varied, as can their tactics and level of magical support.
I've often wondered why there aren't more examples in D&D of vast surface ruins as outdoor "dungeons." Other than Ed Greenwood's Myth Drannor, I can't think of any off the top of my head. I feel like I'm missing some obvious ones, though. Can anyone think of some I might have forgotten?