Thread by @DropTheDie

Alright. I'll bite and try to give some kind of guideline to help you run megadungeons for #dnd5e (well, any #tabletop). General principles, at the very least.
- A thread -

https://twitter.com/Swordsfall1/status/1101324858820980736

There needs to be several reasons for your PCs to explore the place beyond "cool loot." While players LOVE loot, they don't want to, well, have their PC die.
CURIOSITY can be one of those things. "What new aspect of this story waits in the next level/room?"

That is how your PLOT (a dungeon, like any story, needs to have a plot or plot beats) should function. What about this megadungeon will drive my players to go INTO IT not OUT OF IT. "You're at the bottom fight your way out" is lazy and uninspired.

Lets say the adventurers are working jobs for a beloved NPC. That NPC is found dead and the adventurers become investigators (CSI Waterdeep anyone?).
The culprit is a scarred up female Hobgoblin named Ya'viis. Her horde has moved into a labyrinth that was buried by an earth titan

The adventurers scope the place out. "Can't we just collapse it?"
A helpful NPC is great for this aspect. A druid or ranger who has been keeping an eye on the goblins and hobgoblins' comings and goings. "They have slaves in there. Bought with gold by brigands in the first level."

You've got 4 things going for you.
1. Justice
2. Revenge
3. Freedom (for the slaves)
and 4. Gold
You've got 2 enemies - the brigands and the goblins.

THAT'S IT. It can feel boring fighting goblins and hobgoblins and bugbears forever, but they, and humans, are INFINITELY CRAFTY

You take one of the driving forces, and you put it front and center inside the megadungeon. Slaves cleaning bloody armor and weapons watched by bandits.
Immediate payoff - kill the bandits, save some slaves. Slaves know nothing, but a brigand may give details about WAY LATER.

"In the fifth room, behind the iron gate... they made me help them set a trap. I'll tell you how to disarm it if you let me live." This gives them a piece of information about the FUTURE of this journey, something they can keep up with. Like a passcode in oldschool videogames.

Keep on repeating that same equation.
The next room, they find the brigand's staging area, where they drag slaves for the hob goblins. Who's there? Ya'viin. She sees them, has "BONE CRUSHER," her ogre consort smash apart the exit and collapse it.
That DIRTY RAT!

Now you've added a lot of spice into the dungeon. They're on alert now. They could be using slaves to their advantage, they could be setting traps. But no matter what, Ya'viin knows they're there and they know she's inside - somewhere.
The PCs aren't trapped, THE ENEMY IS.

You're putting the advantage on your players and letting them proceed. Do they come back with the city guard? how long will that take? Could she be excavating an exit already? Why did she kill your NPC job giver? Why does she need human slaves - only human? Do they just rush in?

Lets say there are 170 of these rooms in your megadungeon. Split that into 4. (rooms 42, 84, 124, 160) By the time they reach each of these "milestones" they should conclude 1 order of business.
Room 42 - they find a passage that leads to a vast array of stolen treasure.

Room 84 - they finally free the last of the slaves. They were put to work hauling stone out of a great chamber. Dozens died. A statue was in the middle, dozens of stone tomes. Maglubiyet,- the fierce god of the goblinoids.

Room 124 - Now they find the reason. They find the proof they need for their revenge. It was Talik, the swine. A missive from Talik, your friend and ally, offering 5,000 gold pieces for the death of your friend. You'll have his head.. now.. or later? They can ALWAYS LEAVE.

Room 160 - The chamber of Maglubiyet! Ya'viis didn't run. Hobgoblins would sooner die. No, she instead pleased her god. Pleased him with blood and devotion and fury. She has reclaimed for Maglubiyet and is now transformed! And her wrath will be great and terrible. Roll initiative

The whole time they face increasingly difficult odds but these goblins/hobgoblins are trying to STALL. They're throwing Wargs and Barghests and goblins with powder kegs on their backs and ogres and bugbear assassins, but if they leave, they'll NEVER get this holy place back.

And if the adventurers decide they'd rather peace-out on the 42nd room and live out their days? It saves you a lot of goblin grinding! Rejoice! They'll see Ya'viis again. The dungeon isn't a vacuum, treat it the same way you'd treat a castle keep.

And don't start mixing in ghosts and gricks and crazy shit - make the goblins seem interesting and dynamic. Hobgoblin lore is as rich as the tales of King Arthur's Court, man! Do it justice!
Your players LIKE being familiar with their enemy, knowing how to best them.

And if your players can clear 20-30 rooms in a night, and savor the feeling of progress, knowing where the END IS UP FRONT, they'll be more likely to bite in and think of it as an investment, not a slog.

And the BIGGEST TIP LAST.... give your PCs time to talk. Downtime to tend wounds and cook stew and find battleaxes from mercenary companies they're familiar with and use their cool feat that lets them remember shit. Let them feel IMPORTANT and POWERFUL. It's so much more fun.