The Cleaves: An Idea deserving of revival...

A long time ago an idea happened in which adventurers travelling by Dimension Door Transit Service thanks to an NPC Wizard between the Villages of West Cleave and East Cleave which straddled a miles long fissure didnt reach their destination.
They were infact deposited in a megadungeon located in the interdimensional space that all Dimension Door spells took a shortcut through... a Dungeon that Became known as The Cleaves.

The Cleaves were accessible only by Dimension Door error.  To make matters worse, the Cleaves were constantly shifting, so even though you might stumble across an all to familiar room in the Cleaves, the rooms that linked to that rooms would likely not be the same rooms that were there previously.

A dungeon you could access from Greyhawk, or Forgotton Realms, or Mystara by nothing more than the failure of a Dimension Door Spell. A dungeon you could use as a shortcut between those worlds and others... exiting a cave into a dust storm to find yourself surrounded by guys in dusty robes with hoods armed with AK-47s.

With a card deck dungeon, the deck from which you drew your vast megadungeon would be ever randomly expandable by further decks.

And there would be other decks. The Cleaves would come with Trap, Treasure, Monster, and Special Encounter card decks. So an adventurer might encounter a Displacer Beast with natural Scalemail armour covering its body, voices whispering, a key to allow your party to finally Dimension Door out of the Dungeon, a black spider wand, a dungeon room where water flowed from a hole in the ceiling like a waterfall into a hole on the floor and was a perpetual loop, a bed-knob of flying.

And it all became a kind of community built and shared D&D Setting owned by no-one specific, though I went and created a card deck of 101 Treasures of the Cleaves by drawing crappy artwork.