So, my favorite setting for D&D is Eberron, hands down. Eberron (the brainchild of creator Keith Baker) was the result of a contest during the 3.5 edition days, and initially I was skeptical. It's a very different setting from the generic-fantasy Forgotten Realms, and the avant-garde-fantasy Dark Sun.
Instead, Eberron as a setting tries to combine film noir plot elements with pulp-magazine-style action and D&D fantasy tropes. What does this mean?
- Film noir is a term that describes Hollywood crime drama films from the 1940s and 50s; Wikipedia says the films are "stylish... [and] emphasize cynical attitudes and sexual motivations." These are more "realistic" motivations that many characters have had in D&D. The movies had duplicitous patrons using the protagonists as expendable pawns, and shades-of-gray characters; even the most evil villain might have redeeming qualities, and the most saintly character might end up the real villain of the piece.
- Pulp magazines were glossy, sensationalistic magazines that became popular in the 1920s and 30s. Slick and cheap, with lurid covers promising exciting stories, they were a mainstay of popular culture at the time. Many memorable characters were introduced in the heyday of the pulps, like the Shadow and the Spider. While the pulps covered every conceivable genre, from early science fiction to romance, the most popular pulps featured violence and death-defying action, ornamented by some of the great illustrators of the time. A pulp hero could be jumping from one plane to another one week, then dodging death traps in a Mayan temple the next, trying to solve the mystery or foil the plot of the villain.
- D&D fantasy has become a specific genre of fantasy literature. It involves "typical D&D" character archetypes (fighter, thief, paladin, etc.); "typical D&D" races (elves, dwarves, orcs, etc.); finger-waggling, dress-wearing wizards; polytheistic faiths that grant magical powers to followers; and so on. Some elements, such as the presence of murderhobos, are optional but also sadly iconic.
- A de-emphasis on alignments and a removal of definite gods from mundane affairs. Clerics of a particular god can be any alignment and still draw upon magical power, because that power comes from faith, not necessarily from some otherworldly source. This allows the DM all sorts of fun plot elements to explore that are otherwise absent in D&D. For example, a weird cult isolated to one small town in the Forgotten Realms will require a mage-charlatan or some other huckster to explain its apparent miracles. In Eberron, the cult leader who honestly believes the dogma of his religion gets to perform "magic" based on his (and his followers') beliefs. The players would have to work much harder to convince people the cult is a sham.
- A good in-game rationale for psionics. Psionics are the weird power of the Realms of Dreams (Dal Quor) or Madness (Xoriat) where they intrude into the mundane plane.
- A planar cosmology with a lot of breathing room. The planes of Eberron aren't solidly-placed in some Great Wheel; instead, they ebb and flow, drawing closer or retreating according to poorly-understood planar mechanics. This gives a whole specialty of scholars room to argue and develop. Not to mention a field day for astrologer characters.
- A history full of fallen empires and destroyed civilizations and cultures. Humans are not the first culture of Eberron, nor were the elves. The kobolds and lizardmen have an older civilization, based on that of the dragons which is even older. Even the hobgoblins/goblins/bugbears have an older claim than humans.
- Practical extrapolation of magical technology. Eberron has elemental airships and lightning-charged railroads powered by bound elementals, a telegraph system based on magical communication stones, and so on. Most D&D settings are "medieval Europe, but with magic," or "medieval Japan, but with magic;" few people actually ask what effect actual magic would have had on the cultures and technologies of those settings. Eberron effectively tries to reproduce a culture and society different from anything in history based at least partly on the presence of magic.
Anyway, Mr. Baker has stated that he believes Eberron will be a part of 5th edition. A casual read of the Monster Manual led me to quotes from characters from the Eberron setting as well as a host of references, so I think he's right. They intend to release Eberron stuff in a 5E flavor.
Until then, I'll just have to do some conversions of my own.