A lot of this is prompted by thoughts shared on this thread on RPGnet, by Mike Mornard (Old Geezer), one of Gary Gygax's original players. Reading it will give a bit more context into what I'm talking about.
As somebody who read about the megadungeon idea and said, "Wow, that's cool!" and went off to make his own, I have to say that I feel like today's old school modules are going down a path that, while it has its merits in its own right, is neither useful to the megadungeon designer, nor reflective of the old school play that we are trying to get at. Read the RPGnet thread linked above, Mike Mornard makes the main points: a lot of the classic modules are relatively linear grinds because of the requirements of tournament play. And as they were popular, and needed their own justifications in the world, they tended to be given a location, a rationale and a place that makes them awkward fits at best for a megadungeon.
But there are a few products, both older and more recent, that make me think there is another way to approach the module. Two are classic Gary Gygax modules: EX1 Dungeonland and EX2 The Land Beyond the Magic Mirror. These are high Gygaxian funhouse areas that were linked to the original Greyhawk Castle -- in fact, they're really the earliest published levels from that classic dungeon. The other is a more recent product, which I was lucky enough to actually play in: Bottle City by Robert Kuntz. This was another sub-level in Greyhawk Castle, and the product really shows the difference - it's a big sandbox to play in, rather than being a linear, plot oriented dungeon.
What makes these modules so interesting for me is that they validate an alternate model to the classic convention module. The intent is not to have a fully sustained, well plotted and placed dungeon, but something that can be dropped into an existing dungeon with relatively little difficulty of integration. It doesn't have to be Wonderland, since Gygax already did that, but it could be just about anything that is accessible from stairs, a chute, a teleporter, a mirror, a bottle, or anything else you're likely to find in a big dungeon.
But the other thing that sets EX1, EX2 and Bottle City (and the Living Room, which I've yet to receive) apart from other modules is that they are not simply someone dreaming up "what dungeon should I put out next" but real sub-levels from dungeons, which arose not out of commercial or convention needs but were worked up for an actual play group, cleaned up and published. There is something wonderfully authentic about that, the sense that I'm not just reading a scenario the author wrote for others, but an actual level from a well-loved dungeon, which I can add to my own dungeon (or not) because the idea is so interesting.
Who knows? Maybe once I stock it and some players actually get to it, I'll write up the "abandoned temple" sublevel of my own dungeon and put it out on Lulu. But I hope this is food for thought for some of you looking to write a module.