Okay, so I just finished watching the first two story arcs of the anima Sword Art Online. As an anime it was okay, but I wasn't really excited about it. Anyway, the basic idea of the first story arc is that fully immersive virtual reality gaming becomes a reality and the main game is called Sword Art Online. The catch is (spoiler alert!) that the person who programmed the game has rigged it so that once you log in you cannot log out and if anyone attempts to remove your VR helmet while you're in, it will kill you. The only way out for the trapped players (whose real bodies end up on life support in the meantime) is to beat the game--all 100 levels of it. But another catch is that if your character in the game is killed, your VR helmet will kill the real you.
A game based directly on SOA doesn't appeal to me and in practice the real-life part outside the game wouldn't really affect play at the table. But it gave me some ideas for a megadungeon, who which I'd add a bit of City of Ember for extra flavor.
The setting is that players live in an underground city-state, sealed there for ages for some reason (lots of scope for campaign deep background) but now their little world is dying/collapsing/etc. Someone finds a secret way out, maybe revealed by the gods/ancients who put them there in the first place or whatever. But this way out only leads into the big megadungeon which was actually out there all along without anyone knowing about it.
So they have to clear a path through the huge megadungeon to get their people out. This doesn't mean they have to clear the entire thing, just create a safe path to the outside. Monsters have to be killed, enemy groups neutralized, and friends and allies made. The collapse of their home area provides a constant time pressure to get on with it. In the archives of their little world will be many clues to the huge dungeon, most of which were previously thought to be mere legends and myths. Some will mention the various intelligent peoples/races, some the monsters, some particular regions of the dungeon, and several the "promised land" which might lay beyond.
I would run it where there are several "promised lands" mentioned. Some will require the party to actually go all the way there to confirm what it is, others will be revealed by clues found in different dungeon locations. Also, the intelligent creatures/races of the dungeon will have their own legends, myths, and stories. Some of those will confirm the information from the character's sealed world, some will offer different versions, some will be completely new.
I think this is actually a better start for a megadungeon than having the players outside delving in. The problem with delving in from the little starting town or whatever is that there's always the distraction of wandering off to see the rest of the outside world. And there's no pressure to get on with it. They can totally take their time resting up, training, crafting, etc. and go down whenever they're fully rested and ready. The "reverse dungeon" approach would work well for a megadungeon setting tailored for it.