Creativity is a strange thing. I'm not really a creative person, or at least I don't think of myself as one. Most of the time, when I have a kick-ass idea, it's because I've stolen ideas from other people and changed them around to fit whatever it is I'm doing. As a Games Master, you need to steal ideas. The hunger for new ideas is great, and the process of creativity is so incredibly slow that, to fill in the gaps, you need to occasionally beg, borrow and steal. So when someone posts a great idea like The Gigadungeon, I of course straight-jack it from the lovely and talented Zak Sabbath. And when the good folks at Wizards of the Coast write a pretty cool dungeon idea like the Astral Corpse (Open Grave, pp. 120), I try and think of a way I could make that work in a campaign.
Then something strange happens. The two things come together in a way that is unexpected. The whole world is a dungeon, eh? The dungeon is the corpse of a dead god, eh? The whole world is the corpse of a dead god...? Oh that's good. A little squishy, though... The world-dungeon is carved from the bones of a dead god? Better... The whole world is carved from the bones of a dead god. Perfect.
So now I'm working on that. I've got this cute little map of the bones in a human body, and I will be adding a few bones such as shattered wingbones, and horns in places people shouldn't have horns. I don't have much in the way of the setting fleshed out, but I do have a general idea of how I want the campaign to run.
The players begin in the typical fantasy village of Ratella (on the Right Patella, see how clever I am? Praise me.). It's a small farming community, protected by the Tibian Mountains on one side and the thick Femural Forests on the other. Femural Elves live to the north, the Tibian Goblins to the South, and to the south-east are the Fibular Badlands, a place no one goes unless they absolutely have to. It is a place to be feared. Across the mountains are the Tarsil Bloodmarsh and the Metarsil Lava Flats; none of the villagers who have ever gone there have ever come back. Beyond the forest, who knows?
Well, I do, because I'm inventing this world as I go. The Pelvic Grasslands lead to the Sacrum Caveland, which leads to the Spine of the World, a huge broken cliff-face that leads under the Mad King's Cage, huge pillars of stone that jut impossibly high into the air. Small land-bridges lead to the bases of the pillars; this is where the dragons roost and hunt. Atop the Worldcage is the city of Sternum, an enormous place that towers high above and low below the earth. There are ways to reach the city from the Spine of the World, but they are nearly as treacherous as the Mad King's Cage. To the north of the city is the Clavicle Cliffs, huge bluffs that, common wisdom holds, are the end of the world. Walking along these, one can find the Humerous Desert, which isn't very funny at all. This leads to the Ulnar Plains, the home of the dwarves, and the Radial Marsh, the kingdom of monsters. The Carple Islands trail off into the Mecarple Archipellago. From there, there is only ocean.
If one were to climb down the Clavicle Cliffs (which few have ever done), one would find a small landbridge to the biggest dungeon in the world, the Skull (read: Undermountain, but less capricious fun). It is rumored that, somewhere deep in the skull, is an ancient artifact that could revive the dead god that the world is made from, an artifact that many a death cult would pay hefty prices to get their hands on.
I figure the Bones of the Dead God would be most appropriate for the whole of the Heroic Tier, while dealing with the inter-planar fallout of the god waking up would make for a good Paragon plotline, and re-killing said god would take up the whole of Epic. This one plot hook could take you from first through thirtieth with ease. ^_^