I'd love to see a mega-adventure in the vein of Dark Sun's Dragon's Crown, which wasn't just a big dungeon like Return to the Temple of Elemental Evil (OK, one really big dungeon and 2-3 smaller ones). Dragon's Crown took you all over the world (well, the main part of the world which is about as big as Spain) and put you in a couple of very different situations. It was also very world-specific, which I guess is a disadvantage commercially, but it's very nice if you do run things in that world.