Get a functional dungeon down as soon as possible, flesh it out just enough to play, and then fill out the details ahead of the players. It doesn't need to be finished from top to bottom before you can play.
You need to have enough to run with - and the usual rules apply. Side view first, overall plan, etc. But you don't need to do all of one bit before you move on to the next. Once you get enough to play with, you can start going through it in an iterative fashion and add more details as needed and as time permits.
A great example of this - with a thorough explanation of the how-and-why of it - is found in John "Beedo" Arendt's post Developing the Dungeon through Progressive Elaboration .
Having that working framework plus a basic concept will help you decide what goes in, and what stays out, of the megadungeon.
Corollary: Do it before the game, not during.
"Any one of us could improvise a room like the Trader Room - the worst case is that it might slow the game down a little if you need to roll a bunch of dice at the table."
Even slowing the game down a little roll a bunch of dice to find out what's in the room is a terrible thing in my experience. That little slowdown can kill a lot of momentum. That's rolling far, far better done ahead of time. Flesh it out on the fly if you have to, but man, do the "what's in the room?" rolls ahead of time. Your game pace will thank you!
So will your players' suspension of disbelief in the setting. If they know you just rolled up those orcs or that trader, they know anything you say is made up on the spot and not part of a larger whole. They may accept it and run with it, but it doesn't feel like something that was part of a larger whole waiting for discovery. If you write it even minutes before the game begins, it'll flow faster in play and seem more part of the integrated whole. Save the rolling for the stuff that's really determined on the fly - do they hit, do they miss, how much damage, where the orc is standing when you come into the room - and not the stuff that is more firmly set.
Even rolling some dice for the orc's treasure ("they all have 2d6 sp and 1d6 gp") and totaling it up is time you could have spent ahead of time most of the time.
Vary the Theme By Area
An overall theme for a megadungeon is important, but it's also incredibly useful to have a wide variety of places and types of places to explore in it. Because a megadungeon is basically a play area (aka sandbox, aka game world) in a box, you need to provide differences within the portions of it. It can't be a single themed area that never changes, otherwise it's not interesting to progress through and can lead to boredom.
One way to avoid this is variety;
- variety of entrances
- variety of monsters
- variety of levels
- variety of themed areas
- variety of challenge levels.
One good place to start for such variety is Courtney's Megadungeon Checklist. -C's list is meant to be sarcastic, but you can't go far wrong using it as an idea list and a rough checklist of things of interest. You could run a whole game in a dungeon using only what he's got on that list and have a blast. It will lack some originality but will it lack fun? Not likely. Using that checklist you can decide what is too hokey or hackneyed or overused and must be avoided, and what you just haven't tried yet and would like to.
Make Every Level Interesting
Don't start with boring stuff and defer the fun, interesting bits for the lower levels. Start putting interesting things in right away. Put in encounters that are special, enigmas and strangeness worth investigating, and clues to deeper and even more interesting levels below. The upper levels should be interesting and exciting. The lower levels should be a magnet primed by discoveries on the upper levels. In other words, front load the fun.
The proper path is not this:
But rather this:
This is why my level one has a strange temple with a "death zone," headless busts that speak to those who replace the heads, and even a few more undiscovered oddities. Some of these (like the missing heads to the busts) pull the players down and then back up. Even level 1 is weird and interesting. The lower levels are more so, and connect to those even further down.
This one comes out of Roger the GS's The Megadungeon Paradox
Thanks to all of my inspirations for this post - I couldn't have written it without your words.