At any rate, the idea has stuck with me all day since I read it on Tim's blog. Off the top of my head, here are some initial ideas that might have some potential in the fantasy and sci-fi genres (I'm less familiar with fiction and games in other genres, so no ideas jump readily to mind; I'll let better minds than mine deal with those).
A massive city. Each GM adds in his own quarter/neighborhood (or even multiple neighborhoods). The city could have above ground intrigue as well as below the city sewers/dungeons. This would require some common agreement on overarching political system, religion, etc. but it's probably doable.
A world of islands on a vast ocean. Each GM can add additional islands at will, creating an ever-expanding milieu as needed. Very flexible as one island could be far enough away from another that there wouldn't necessarily be identical cultures on each one (or there could be leeway for slight variations).
A world of independent city-states. Like with the island option, each GM can add additional city-states at will, creating an ever-expanding milieu as needed with plenty of leeway for variation without actually hitting "inconsistency."
A megadungeon. Each GM adds in his own sub-area of each level. There might have to be a common city or town nearby, again with an overarching political system, religion etc., but again, this could easily be negotiated among the GMs I think.
A Bellairsian South Kingdom. "If you looked at a map of the South made in Prospero's time, you would think it was a badly done and rather fussy abstract painting, or the palette of a demented artist. You would see blotches within splotches within wavy circles; you would see shapes like ladyfingers, like stars, like dumbbells, and like creeping dry rot. All this was the fault of Godwin I (Longbeard), the first King of All the South, and the last to hold any real power. He divided up the kingdom among his sons, and they did likewise, and so on. Primogeniture was never established, so eventually the south became an indescribable conglomeration of duchies, earldoms, free cities, minor kingdoms, independent bishoprics, and counties. These little worlds were often the size of small farms..." (John Bellairs, The Face in the Frost). Every GM could easily just plunk down his own small realm(s). You'd still need some common factors (like religion). But very flexible and workable I think.
Sea of Planets. Maybe the PCs are employees of a private interstellar Salvage-Rescue-Security company or something along those lines (thus avoiding a military hierarchy, so no one PC is in command). Maybe the PCs get sent to various planets by their (distant) boss on diverse missions such as recovery, rescue, investigation, defense, infiltration, raiding, etc.). Each GM can create planetary systems of infinite variety at will. Similar to the fantasy "city-states" or "islands" motifs, but in space.
Massive Space Station. Kind of like the fantasy "massive city" motif. Deep Space Nine or Babylon Five on steroids. Lots of different sections of the station, races, etc.
Massive Cosmopolitan City (in this case, literally – cosmos + polis) on a planet in the middle of intergalactic trade routes, etc. Similar to the "massif city" fantasy motif.
Abandoned Dyson Sphere. This could be turned into a stellar megadungeon. Maybe not with multiple levels vertically (though I suppose there could be). But it would just be so darned big you might not need levels.
Just the broad, rough ideas that immediately jumped to mind over the last twelve hours or so. I'm sure there are other possibilities.