A brief, and largely pointless, paean to the joy of incremental emergent complexity (aka, the power of "How did that get there?").
Ideas are odd. They have a tendency to grow beyond their bounds if you let them. You have to be careful of the things, or they'll grow, ramify and insinuate themselves like briers (kudzu to yon Yankee devils).
Consider if you will the example of the cabal of Masked Sleepers on the 1st level of the Vaults. Now, I didn't consciously write these chancers into the Vaults, they just emerged from a random idea which was worked on by the needs of the game and the internal logic of the place.
The Masked Sleepers started out as a quick, lazy reference on a wandering monster table:
"19-20. Masked Sleepers - treat as zombies".
Simple enough. Just your bog standard zombies with a palette swap and Mycenean death masks (I like Time Bandits: who doesn't?), designed to fool the players into not immediately making the obvious "Brains!" ===> *yawn* "Turn undead" connection. No change in stats, merely a fluff modification.
A little thought and some judicious idea theft later (In Nomine: the ironic Hell of Nybbas, a place in which each sinner focuses obsessively on a fetish object to the exclusion of all else), and the Masked Sleepers started to become something more. The image of eight gently rocking Sleepers standing in a fixated circle around a glowing shewstone in an otherwise darkened room is one of the more memorable images of recent sessions.
"What are they doing?"
"No, just humming. And swaying a little. They're not even looking at you..."
With the masked sleepers established their presence gradually expanded to touch other, already established, elements of the Vaults. The otherwise randomly parachuted-in Oneiromantic Hall ("Glass walls swirl with multi-coloured smoke. Stagnant pool surrounded by benches in centre of room. Elaborate curtained beds in wall recesses. Anyone resting in a bed has 50% chance of dreams cryptic but true") on the first level of the Vaults became the means of their creation ("Dying person placed in bed arises as Sleeper 1d6 turns after death").
Once they'd wormed their way in there, there was no stopping the gradual encroachment of the blank-eyed mumblers (not related, but fun). I ended up having to write a whole new section of the Vaults (thank goodness for the One Page Dungeon template, and the Empty Room Principle). The flaying rooms, skin frames and ossuaries of the Masked Sleepers gradually got linked in with the tribes of scavenging humanoids (tologs, chokers, goblins) on level 1, and with the Gnoll/Ghoul tribal areas on level 2.
Likewise the half-dozen "I dunno" blank rooms that slowly morphed into the Oneirist's Sanctum, and the - as yet unexplored while I figure out a fun twist - Carceri of the Unreconciled sub-level. Entirely without conscious volition on my part, a simple mob of slow-treading lost souls became a mini-motif of the upper Vaults in their own right. Not bad for a throwaway comment.
That said, I still haven't done anything with the Rook Seers I mentioned in passing, ooh, yonks ago now...