Here are my predictions for what WotC will actually do with Dark Sun, keeping in mind the constraints they operate under (specifically, Dark Sun must be fully compatible with all core D&D books, and reasonably compatible with the re-released Prism Pentad).
Races: All of the standard PHB races will be included, but each will get a writeup explaining how it fits into the Dark Sun world. Eladrin will be "city elves" as Tzarevitch suggested. Dragonborn will be merged with dray; they will still be called dragonborn in the rulebooks, but it will be noted that "dray" is a commonly used term, and their origin story will match that of the dray. Tieflings... well, frankly, I dunno what they'll do with tieflings.
In addition, a couple of the old Dark Sun races will be added. Definitely thri-kreen, probably muls, possibly half-giants.
Classes: Divine classes will draw their power from various entities, including elemental forces, sorceror-kings, the Dragon (if there is one), Dregoth, and so forth. There will be appropriate Channel Divinity feats for each. Other classes will be pretty much as written. The PHB wizard class will fill the preserver role, just as in the original.
In addition, a new base class will be introduced - the defiler. Defilers will be arcane controllers with a lot of debuffs and a focus on "screw your friends so you can screw your enemies worse" powers, much like the dark pact warlock in FR. Their signature class feature will be a defilement power that they can use to "power-up" in various ways. Each time this power is used, it triggers a close burst which causes a short-term debuff to all creatures and destroys plant life.
The World: In order to preserve compatibility with the novels while giving themselves freedom to modify the world as they see fit, WotC will go the Spellplague route - that's not to say they'll have an actual Spellplague, per se, but there will be some titanic cataclysm that reshapes the face of Athas. The 4E Dark Sun setting will be several hundred years, maybe as much as a millennium, after the Prism Pentad.
Most of the old cities and sorceror-kings will still exist, but perhaps in altered form. The "big bad" of Athas will either be Dregoth, or a resurrected Dragon. If the latter, the aforementioned cataclysm will almost certainly be connected to the Dragon's return. (It's also possible that Dregoth will complete his dragon metamorphosis somehow and then go on a rampage just as the original Dragon did.)
The History: The new Dark Sun setting will avoid giving more than hints as to the history of Athas. Although the setting will be fully compatible with the events of the Prism Pentad and all the existing Dark Sun lore, it won't actually contain much if any of that lore; there will be references to a few of the Prism Pentad characters and locales, but no overarching timeline. Thus, newcomers to the Dark Sun world, or old fans who didn't like the lore introduced by the novels and the later supplements, will be free to write their own history.
Cosmology: The Shadowfell will fill the role of the Black, and the Astral Sea, which will be dominated by psionic rather than divine entities, will fill the role of the Gray. The Hollow might be an astral dominion or it might be a "domain of dread" in the Shadowfell. The Feywild will exist only in scattered pockets. The Elemental Chaos will be essentially unchanged, though it may or may not contain an Abyss.