Michael Curtis, author of the eminently excellent Stonehell, had an awesome idea - using hangways to let visitors enter and leave his campaign.
Standalone, it's a neat mechanism to explain the sudden appearance of drop-in characters. He's got 3 other ways listed on his blog post.
I think it has more potential as a universal mechanism to travel between megadungeons. Anybody running their custom megadungeon, can just drop one in, and walk their characters off into some described spot in Stonehell with no additional work necessary. It's a very cheap and easy way to integrate somebody else's work into your unique piece.
Taking that idea to the next step, if a bunch of megadungeons are published with hangways, then you've got a whole series of unrelated products you can plausibly have players exploring. Yeah, any DM could do this anyways, but having the hangways right there in the book as published, with rooms designed to contain them in a meaningful way, just makes it nicer.
I begged Michael to let me use the hangway concept in my megadungeon, and he graciously allowed me. So there will be one or more in the Anomalous Subsurface Environment.
A little standardization about which levels are good for hangways, so that monster threat across the two sides of the hangway isn't too out of whack, would be a good thing I think (e.g. "Hangway A would be on level 2 or 3 if it exists, Hangway B is levels 5/6, and Hangway C is 8/9"). Another thing that would be cool is a logo people can put on their products to indicate hangway-compatibility.
I'm feeling totally inspired by this.
On a related note, who else is working on publishing megadungeons? I know James Maliszewski is publishing Dwimmermount, and Blair is doing a Planet Algol megadungeon. Is anybody else there going to be putting out product?