The four personality traits of your dungeon: Something something brigs, you know, because there's the personality trait but also the pirate word for dungeon? You know? There's a joke here I swear I just can't find it

Trait One: Architects

Every dungeon starts with someone who has no intention of building a dungeon deciding to start an ambitious project. This architect has their own style, which will inform the final structure considerably, and I think it's useful to jot down a few notes about them. Note that I'll be referring to the architect as a person, and in some cases it will be an NPC that lives (or lived) in the game world, but more often the architect is just a nameless member of some faction. 

Consider the architect's style, but also their resources and needs. Dwarf-made structures will tend to be angular, but also incredibly spacious- Both to make it easier to move heavy equipment around, and because they're using what they're mining, so the size of the room corresponds not to any particular aesthetic concern, but the shape of the mineral veins. Dwarves might also have a preference for putting treasure at the center of their vault, preferring obvious, sturdy defenses to more evasive ones. 

Trait Two: Purpose

With a few exceptions, nobody starts a megaproject first and then decides what all the rooms are for later. Temples will be different than fortresses will be different than prisons. Think about both the structure's purpose as a whole, and all of the sub-functions that might be useful in providing for that function. For example, a temple to the wind god might well have a vast aerie, a corresponding enclosed forest to serve as a squirrel farm to feed the birds of prey, a steam system for keeping the forest and the aerie warm during all seasons... 

Trait Three: Calamity

A structure isn't a dungeon; It must be ruined. Something killed the architect and fouled the purpose, and the form of the calamity will generally be obvious to the players and provide important clues about what functions might still be functional, or dangerous. If there was an invasion, expect skeletons strewn about, especially around the site of traps (smashed bones are a great heads-up about a falling ceiling trap). If the calamity was something like a plague or a famine, there might be barricades and sad little diaries written by the last handful of the original occupants.

Trait Four: New Tenants

A ruin isn't a dungeon- it must be repopulated! Who came creeping in to set up shop, willing to pay rent in blood to convert this dangerous wreck into a home? What do they like and dislike about it? How do they feel about the calamity? Consider the Sacred Bath, overheated to boiling to use as a giant stewpot for some gnolls. Consider monstrous spiders hunting the royal eagles and spinning webs between the aeries. Consider ogres smashing the hell out of every trap in their area.

1d10 Random Architects:

1. The Empire That Had Lots Of Money and Mundane Resources

2. The Dwarves

3.  Elves

4. Druids

5. The Evil Empire

6. An Outsider (roll to see which plane, I guess)

7.  A mad wizard

8. A Dragon or other massive beast

9. The Empire That Had Lots of Magic But No Goddamn Sense

10. Underdark

1d10 Random Megabuildings:

1. A temple

2. A tomb

3. A fortress

4. A city

5. A prison

6. A research lab

7. A university

8. A labyrinth

9. A weapon

10. A palace

1d10 Random Calamities:

1. Plague (fever)

2. War (Mundane)

3. Plague (Myconic)

4. War (Planar)

5. Plague (undeath)

6. Plague (madness)

7. Famine

8. War (Atomic)

9. Flood

10. Mutiny

1d10 New inhabitants

1. The original inhabitants, perfectly preserved by design or accident, either because they're immortal or they were frozen or something. Either way they have some questions. 

2. The original inhabitants, imperfectly preserved. Half-vampires, half demons, half-machines, half plants, whatever. 50-50 chance they yearn to die.

3. "Mindless creatures" Oozes, hunger orbs, certain kinds of ghost.

4. Beastmind creatures. Wyrmwolves, Octojellies, a hydra, gnomes. You can't quite negotiate with them, generally, but at least they feel pain and have preferences. 

5. Old Ambient Wildlife. Plants like the Scuttling Lotus should fall under creature types- This is for stuff like Very Poison Ivy and Tunnelsbane which functions almost as a trap or architectural feature. 

6. Squatters. Goblins, humans, vampires. People who just saw the ruin as free real estate. 

7. Calamity: Whatever took out the dungeon was pretty cool. There's probably someone around that was either part of it (see: zombie plague), or possibly modern enthusiasts trying to recreate it, worship it, prevent it, etc. The gnome 

8. Scavengers: Like squatters, except they're here to extract value and then leave. At its most basic, it's a mixed-race "adventuring party" here to get "treasure." But it could also be something like a detachment of elves trying to collect the ten Soulstones hidden inside, or a family of Mycon collecting (and converting the alive into) corpses to send back to the Main Hive. 

9. New Ambient Wildlife: If there's a family of squirrels stealing insulation foam from the Ancient Egg Incubation Chamber, that's wonderful.  

10. Robots. Golems. Whatever you want to call it, it doesn't quite have a MIND per se, but it's not an ooze, and IDK. Run with it.