It will be Dark Sun or Dragonlance, and I hope Dark Sun.
Ravenloft doesn't need a specific setting book since they can essntially merge Ravenloft into a future Shadowfell sourcebook. Which works for me.
Likewise, Spelljammer and Planescape probably will never get a dedicated setting book but will be spread out over the planar sourcebooks. With Sigil in the DMG2, Manual of the Planes and the upcoming Elemental Chaos sourcebook among others, you are covered.
Mystara, Greyhawk, and Birthright are too vanilla fantasy and they already did Realms.
Dragonlance is vanilla fantasy but has an active book publishing line to tie it with.
Dark Sun is my prediction and my preferred choice. As far as them not doing justice to it, well the only GOOD Dark Sun setting product was the original one before Troy Denning blew up the world with the Prism Pentad.
The Revised Dark Sun setting that came out later was garbage. The Dragon magazine revisit by David Noonan had the right idea by setting the timeline far enough in the future where the Prism Pentad stuff didn't matter, but it also had a few issues, like trying to bring in paladins and stuff.
With 4e that doesn't bother me as much since the 4e paladin class is more of a holy warrior and less a crusader for good. With suitable reflavoring, I'm ok with the 4e paladin in Dark Sun.
Personally, if I was designing Dark Sun, I would make a few drastic changes. I would have no Crimson Savanna or biotech halflings. The world would be all desert with a few isolated jungle regions in mountain areas where rainfall might occur. But, I would drop the whole oceans of silt thing and actually have salt water oceans. It just makes more sense from a scientific standpoint as to why a desert world would still have breathable oxygen (gigantic algae blooms) unless you intend to rip off Dune and use oxygen generating sandworms or something. There also has to be enough rainfall and water to keep subsistence farming viable.
I would also bring back Oronis of Kurn and keep Tyr as a free city. I think one Avangion SK and the free city would provide an adequate couple points of light in an otherwise bleak Road Warrior style fantasy world. In fact, bringing back Oronis provides a ready made explanation for paladins. They are Templars of Oronis. Dragonborn would be Dray. Tieflings I would drop altogether. And the other races would all need to be reflavored appropriately. You also need to add Muls, Half-giants, and Thri-Kreen.