So, here's my first cohesive idea for a megadungeon.  I'm sure it will morph as I work on it, or I might throw the whole thing out and do something different. I like that it evokes the feel of many layers of the dungeon, that each section will, in theory, build off the themes of others.

Citadel of the Black Wizard
A Pathfinder 2e Megadungeon for players of level 1, spanning all the way to 20^th level. Shoot for the moon, right?

Dursil, the Black Wizard, is sometimes called the one who will devour everything. Yet, from the heart of their grand castle it might seem a joke. But beneath their castle, the universe begins to unravel, as though it were being entirely consumed, down to the very rules that keep it together.

This destruction draws all manner of dark creatures to the citadel, and with them come their followers. Others follow in turn. Chaotic creatures with no agenda but to bask in the relative safety provided by their more destruction-minded bretheren. Adventurers whose goal is no more complex than putting an end to one of the many horrible creatures that thrive here – or even Dursil themselves.

Sounds exciting! Surely some PF fans will want to pick that up and check it out! 

That's the hope!  Framework has been coming together with some neat ideas. :)

I have little experience with megadungeons, just two campaigns I ran that were headed in that direction. One was a free form dungeon set in a vast series of caves. It had about a dozen different zones and I researched all kinds of geology to make the areas feel different from each other.

The other was an extra-planar library, a repository of all the information collected by the original builders of the cosmos. The first arc was about stumbling into the library by accident and then finding a way out again, brushing up against some of the factions and storyline along the way.

I'll probably chase one of those two ideas. What a big exciting project!

Edit: found the first map I made of the Colossal Caves. I have no idea what the numbers mean. Maybe they are depth indicators?

So, I've dusted off an old random dungeon generator script I had written, to give myself a starting point for the many hundreds of rooms I plan on creating. I never really polished this tool very much, and inconsistencies abound. The first dungeon I generated today was:

Although it appears old, this building was erected only just yesterday by forest elves. Nobody remembers why it was built. It has seen many changes in ownership over the years, by man and beast alike. Colonial Humans intent to use it to defeat rat.

Ah, random generation...

Time to update my 5-dimensional mega-dungeon

Not to be content to settle on my first idea, I've had another that is more interesting than the basic "a wizard's tower"

The Blood God's Prison Ship

Every world has stories of those who came before. Sometimes, these stories are of cultures fallen just generations ago, to war or famine or strife. Sometimes, they are older – people who lived long ago and built monoliths, and eventually faded away. But sometimes, these stories are of people from elsewhere. Other planes. Other times. Other worlds. These Elsewhere People visit, leave impressions on local races with their vast power and knowledge, and then depart once more.

Usually, these Elsewhere People visit because they have a purpose. And in this case, that purpose was at once kind and careless. They remained long enough for stories to be written of them, placed a new star in the sky (called the Visitor’s Star), and then departed as mysteriously as they arrived. This was not a true star, but prison for a demigod. They could not allow them to live freely, for fear of the destruction they would cause. They could not allow them to die, or else they could ascend to true godhood. And they could not imprison them on their home world for fear of someone setting them free. So, they created a prison, aboard a satellite, to around this distant world, for all time.

But the longer the scale, the less you can plan for. It has been hundreds of years now, and the prison’s systems was struck by an errant comet, knocking it out of orbit and sending it eventually down to the surface of the world. It will break up in orbit, and scatter across the surface of the world.

The ship had many “ward” modules, and was able to rearrange its shape to a limited extent. Each ward module had a combined purpose of keeping the blood god contained if it were to escape, keeping anyone else out who wanted to enter, and housing part of the station’s systems, and part of the key to the blood god’s cell, so that to truly escape . As such, even if one module is damaged, the station would still function… except that some modules are more important than others. There was only one control module. And there was only one orbit module, and it was this which was struck, exploded, and caused the station’s fall.

My current plan is to build a Heroes of Might and Magic style affair. It translates perfectly to a dungeon, just replace dungeon with continent, level with nation, and room with province and the logic holds true.

After one thousand years, the ancient lich has broken free from their black prison beneath the earth, and now hordes of undead swarm across the land once more, surging up from the ground and taking the realms by surprise and subjugating them in a manner of days. You and your companions, a band of mercenary commanders, are in the eye of the storm of conflict, where the undead are there weakest. Can you muster your forces and drive back the legions of the lich from the free nations, then take the fight to the underworld and vanquish the ancient evil once again?

Trying to hash together a custom system, combat is pretty clearly going to be its own minigame on a small grid of 12 spaces or so. Big emphasis on the unit defining your primary combat abilities while your commander is more utility and out of combat stuff. Additional units would be more powers than additional bodies on the board to keep gameplay from just turning into a wargame and make stuff flow faster.

There would be multiple nations to free with all kinds of locations, units to recruit, ancient treasure to find and allies to make. Eventually doors to the underworld would open up, allowing sojurns down into the lich's own realm to take the fight to the heart of their dread power. Players know some broad landmarks at the start, capitals and such, but plenty of surprises to discover along the way.