As part of an ongoing Campaign Design/History I've been assembling, our group - the Foaming Flagons decided to create a single world map that we will adventure in. John was kind enough to assemble the main layout, with the only pieces missing being Harn and The Land. I added those in, and currently:
John has his 0E campaign in the Wilderlands
Brett has his 2E campaign in Faerun
I have my 1E campaign in The Land/Harn
The benefit of such a world is that there is the very real
possibility that the players from different campaigns may well meet each
other/adventure together. But more-so was the concept that we all
agreed upon of a "Living City within a Dungeon" complex.
The group in my campaign is at a point that they might well descend into one of these Living City levels within Castle Zagyg (depending on which of the 3 sets of stairs they choose).
Lastly, in the very near future, I'm gearing up to run a Middle
Earth campaign set during the 2nd Age - just forget any reference to the
ring and you've got the correct mindset.