While I've been itching to run some classic modules and try some of the new classics that are coming out of the OSR, I've got this idea in the back of my head for writing my own MegaDungeon. The problem is the concept. I know I could go the easy route and use the "Mythic Underworld" concept, but I want something more detailed, more "realistic". I've come up with a few ideas, but I think they all have drawbacks.
"The Necromancer beneath the city" The city once had a large underground dungeon in which it held its prisoners. Very little of it has been used since the Mad Mayor was overthrown decades ago. In recent months, public executions and exhiles have ceased, and other prisoners are starting to disappear. No one in the public knows exactly what has happened to these prisoners. In truth, they have been thrown into the old Dungeon in groups on a weekly basis. A powerful Necromancer has moved into the depths of the dungeon, and has threatened the City fathers with devestation if they do not provide him with "tribute" in the form of living bodies. Once thrown into the dungeon, these prisoners are rounded up by the Necromancer's minions and taken to holding areas where they are later used in various experiements.
The individuals in an adventuring party are thrown into the dungeon for whatever reason (actual crimes, politcal revenge, outlawed magic or religious practice etc), but before they can be rounded up by the Necromancer's minions, they encounter an old elf who has lived in the dungeon since the time of the Mad Mayor. He leads the party through a hidden tunnel and provides them with some basic supplies. From here the party can explore the dungeon, encountering the Necromancer's minions, many unusual experiments, forgotten areas from the time of the Mad Mayor, and eventually the Necromancer himself.
To extend the adventure even further, the Necromancer is taking most of the prisoners and turning them into zombies. He is then selling these zombies to a political power in the "Underdark" to create an army. Either before or after defeating the Necromancer, the party can venture into the "Underdark", ala the D1-3 series.
The drawbacks here are figuring out a reasonable/realistic base of operations. Without a way to get out of the dungeon, there's really no way to sell loot or get supplies, which creates an interesting situation. The best solution I can come up with is providing a few secret tunnels from the Dungeon into various parts of the city. Without access in and out of the dungeon, it makes for a really good campaign idea, but it doesn't lend itself to the MegaDungeon play style all that well. In a way it reminds me a little of the situation the a party finds itself in at the start of B4-The Lost City, or A4-In the Dungeons of the Slave Lords.
"The Endless Catacombs" The city has buried its dead in a gigantic labyrinth of catacombs for milenia. Adventurers can enter the catacombs for various reasons including: Looting treasure, catching tomb raiders, clearing out invading mosters etc.
Stole this idea from Glen Cook's 2nd Black Company Book, Shadows Linger. The problem here is that if it is one gigantic catacomb, the deeper areas are actually the newest, and if they are still being used for their intended purpose, there wouldn't be much adventuring activity there. This also tends to be heavy on the Undead.
"The Mines o' Death" The mines are a combination of naturally occuring cave complexes and mines that have been built over the centuries. Various powers have controlled the mines over the years, but they have been abandoned by civilized races for many centuries due to a lack of production, dangerous conditions, and climate change in the region. The mine is now inhabbited by multiple humanoid factions, as well as all sorts of strange creatures that resided here or were brought here by the creators of the mines.
This is a pretty straight forward concept but seems to have a lot of potential. I think it has much in common with Moria, mixed in with the Caves of Chaos from B2. I envision the complex having many, many entrances spread out over a few miles, rather than the few dozen yards in caves of Chaos. The caves and mines will interconnect in various interesting ways, and provided for a huge variety of areas. I like this better than the abandoned Dwarven city model because it allows for more variety and creativity. One of the major drawbacks in this one is that it lies far from civilization (probably a week's journey or more to the nearest outpost) making it an unlikely location for lower level adventures.