[A combination of three different edits of my submission to Wizards of the Coast for the create a campaign setting contest that led to the creation of Eberron.]
Core Ethos Statement: On a world where the elements are the enemy and the gods are not your friends, mankind fights for redemption.
Who are the heroes? The lone, scattered clerics, listening for the faintest whisper from their gods, and the monks sworn to protect them. One example is Selornim Vass, High Priest of Wokond. Selornim is in closest contact with the God of Mercy, and in perhaps the best position to broker peace between man and the gods. The druids who swear fealty to the Element Kings, but scheme in private to aid mankind against them. The fighters who protect the borders of their principalities from constant hostile threats. One example is Torvol Drosk, Warlord of Parumbrol. Torvol has defended the southern border of Parumbrol from the monkey golems of Vodania for years with just a small army and his elemental sword. The paladins born to a destiny the gods had left for man centuries earlier before the Divine Exodus. The rogues who slip into the long-sealed temples of the gods and steal back their artifacts for their current worshippers. The wizards who learn the ways of magic in secret guilds so they can one day master the Elemental Planes. One example is Grand High Transmuter of the Principality of Harumbar. Gederand is the most knowledgeable man on the planet of the schemes of the Element Kings.
What do they do? At basic levels (1-3), the heroes foil the agents of chaos -- such as the elves, the gnolls, and the lermeshurs (lemur-men) -- as they work to break alliances and disrupt the principalities. They struggle against a cold, hostile world environment full of wild lands and savage monsters. At the mid-expert levels (4-7), the heroes gain the notice of the Element Kings as they begin to fight back against the elemental golem hordes and become forces to reckon with in the world around them. At the high-expert levels (8-12), the heroes gain the ire of the Element Kings by finding allies from the Elemental Planes, and their delvings into the underworld begin to produce evidence of a previous age when man and gods were still close. At the master levels (13-18), the heroes can begin combating the Element Kings, who for 600 years have ruled the world as their own, on their home ground, winning the attention or favor of the gods. At the epic levels (19+), the heroes can begin reuniting the gods with man, winning redemption in the eyes of the gods that their ancestors abandoned, ending the reign of the Element Kings over the world, and restoring the kingdoms of old.
Threats, Conflicts, Villains: The halflings are slaves and the race of humans are peasants in the eyes of the Element Kings. The dwarves are free, though they revere the Element Kings, and are busy vying with kobolds for their layer of the underworld. The elves scheme against mankind to keep them disorganized, because men have had their time to be the dominant species and next it could be the elves' turn again. Orcs are common and control many towns and much of the trade. Bugbears, ogres, and dire elemental animals prowl the wild lands. The gnolls are direct pawns of the Element Kings, disrupting the alliances of mankind for their own ends. Worse still are the elemental golem hordes, monkey- and ape-shaped embodiments of the elements of nature. The underworld holds secrets and wonders of the past age, but is also home to fearsome races like aboleths and beholders. Finally, heroes will seek allies on the Elemental Planes and push their battles to the home ground of the kings themselves. There are many Element Kings, “big bosses” of astonishing variety of powers.
Nature of magic: The world is surrounded by such a density of Inner Planes that the Outer Planes have almost no contact, and the world itself is infused with elemental magic. The gods are distant, and have but a few score clerics each at any time. Demons, devils, and other outer-planar beings are virtually unheard of. Undead, in a traditional sense, do not exist. They are empowered by the Elemental Planes rather than a Negative Material Plane.
What's new? What's different? Once, the gods had an active hand in guiding the world, but 700 years ago the gods left the world, telling mankind that they would return. It was called the Divine Exodus after they left, because the world was a cold, desolate, and dangerous place without the intervention of the gods. Then the Elemental Kings contacted the leaders of mankind and told them that the gods had left for good, and offered to protect mankind in a dangerous universe. And so man began to pay tribute to the Element Kings and lost their freedom. The Element Kings each rule an Elemental Plane - but not ones for air, earth, fire, and water. Rather, there are over a hundred Elemental Planes corresponding to coal, tin, iron, etc. he Element Kings have no intention of giving the world up even though the gods have long since returned. And the gods, miffed with man, aren't hurrying to reclaim it.