But here's what I've been thinking.
There needs to be a town nearby, because a dungeon is a source of income like a new gold or silver mine, and people will build shelters and houses to be near the boom. In this case, there's already a town nearby; a bit of tweaking with the results of a random fantasy name generator gives me Rakelia. The name sounds Roman, so I've decided that it belongs to a not-quite Roman Republic. Romans generally built forts and then settled with soldiers, so I can decide that there's a stone keep that has older buildings inside, occupied mostly by citizens, and that outside the walls are the non-citizens; slaves, adventurers, and services.
The dungeon itself is (was?) underground, and was opened up by a fissure. I've decided that it's plate tectonics, which means that it looks a bit like this:
|Imagine that the there were a handful of stone rooms half-opened by that fissure near the bottom|
And it has the added bonus of meaning that the dungeon is going to be split along this crack and that there are really two sub-dungeons now that connect and diverge along the split foundations and crumbled walls and ceilings. Going up will be just as much "forwards" as going down, sometimes, and might even be the only way to bypass a particularly formidable barrier. Crossing the center fissure, at the narrower parts where the floors haven't collapsed too much, is possible, and sometimes the party's going to need to leave ropes down and scale to the next level without stairs or ramps.
For the denizens of the dungeon, the ancient and modern come together as spontaneous generation and Gygaxian naturalism come together. Lesser monsters and animals spawn wholesale in the dank dungeon grime, which provides food for the monsters that spawn biologically. Some more dangerous monsters are an open question as to their generation- are goblins profligate breeders or do they simply appear in untravelled and shadowy halls? Do otyughs spawn from underground midden heaps or do they somehow mate, tangled rubbery limbs and all?
I've decided that there are multiple entrances from the surface, as befits a dungeon of this size, but haven't nailed down exactly how many there are. There are are at least two, which I've named the Left Hand Path and the Right Hand Path, and they are both near the town. They require a bit of scaling to reach, and probably require a rope. The Right Hand Path is the one nearer the players, and the Left Hand Path will require the players to find a way around or across the gap.
I'm working on the wandering monster tables, and I haven't started mapping. The plan is that on each level there's a small handful of "set-piece" encounters that are more or less static, and the rest of the dungeon is handled by wandering monsters or random events (depending), the overarching environment (a little different for each level, and tied in with the "theme"), and just plain empty rooms.
A quick example of what I'm talking about off the top of my head:
|With glorious orcs, of course|
Level Six: High Alert
Environment: The orcs in this level have fortified multiple fall-back positions and the patrols are faster. Roll for wandering monsters twice as often. Orcs that are losing a fight will attempt to retreat to their Stronghold. Denote the escapees and include them in the Stronghold.
Stronghold: An enormously fat Orcish sergeant with a flaming blade is directing his soldiers. The archers are in elevated towers made of wood spiked with crude nails and provide supporting and accurate fire. The soldiers fight in a box formation, with the sergeant in the second row bellowing commands.
Room 1: Orcish ambush, they have set up spiked logs to direct foot traffic into a "kill zone" that will be feathered with their archers and repulsed by spearmen
Room 6: Prisoners, chained to walls. Most are recently dead or dying. All are wearing ragged clothes and bruised/beaten.
Room 15: Midden heap, mostly garbage but sometimes cool things get tossed in, too
Room 18: Hole in the ceiling leads to Level Five.
Random Events (2d6):
1-2: Dogs barking and snarling in the near distance- it's orc beastmasters and war-dogs!
3-4: The players stumble into a trap; falling rocks, poison darts, covered pit (dumps into next level), leg trap
4-5: Organized patrol discovers the party: 1d6 orc warriors and either a sorcerer or a priest accompanying
The random events will probably be in a table, so I'll be able to say how many times they're supposed to happen by marking them with a  or ( ), so you could print it out and mark it with a pencil if you're playing at home, but my plan is to save it as a .png and doodle on it with paint. If I randomly roll something that's out of "charges," Ican either pretend it happens again anyways, roll again, or give them a break.
The important thing is that each time the players go back in the dungeon, if I write it this way, it's easy to see what will refresh and replace itself, and what won't. It's easy to say that even if the players carve out the heart of the Orcish commander and clear the Stronghold there are still some wandering orcs looking for treasure or food, or that there are some fresh traps that haven't been sprung by anybody yet. This means that even "cleared" levels are still plenty dangerous and more than capable of giving any party a run for their money.
So it should be fun. The plan is to write enough in advance that I know roughly what's going on and what's where to give the players a megadungeon experience inside of Dungeon World, which is going to be tricky. I might write the environment as Dungeon Moves instead (so that the Dungeon might make a move if the players roll a 6 when they're not in combat to simulate wandering monsters, or maybe in a particularly rickety floor there might be a Dungeon Move that goes when the players crash around, roll +Party Size and that'll drop one or two of them into the next level, where they'll have to deal with whatever's one floor below them, or something).
I'll need to draw maps soon; I hate drawing maps.