Wow, I stirred up a hornet's nest with that, didn't I? Anyway, here's the deal based on what I know, what I think I know, and what I've heard from various people.
Here's what I have confirmed:
Kender and Warforged are going to be in the next Playtest Packet dropping in a week or three.
Keith Baker has had informal talks with Wizards of the Coast working on Eberron, but there's nothing set in stone as WotC is still deciding on setting support.
Margaret Weis has stated that she would not be adverse to coming back to work on Dragonlance and gives her blessing for the setting to come back even without her.
Ed Greenwood is writing not only a column on Forgotten Realms (and has been doing so for Dragon Magazine for I think two or three years at this point), but he is writing the first novel of the Sundering, which is the big edition change event for Forgotten Realms this time around.
That's it for confirmed stuff. There's a few other things that's come up. First, the current editor for online content for WotC is Miranda Horner, whose first jobs for the company were editing Dragonlance and Ravenloft. Now, Chris Perkins is the one who actually makes the decisions on what to print and Miranda pretty much just makes sure there's no typos and everything looks nice on the page, but it is something to keep in mind. (I've got an interview with her that I should have up in a week or two, depending on how a few things pan out).
WotC putting their back catalog up as PDFs is a pretty big indicator of how they're treating this edition, almost as much as how the edition itself feels in the current playtest. They're paying a lot of respect to older fans while still trying to make the game accessible to new players.
And that's the important thing: Paying respect. A LOT of players who jumped ship during 4e era did so because they felt the game had moved too far from what they felt D&D was, and that WotC wasn't paying respect to the material. Do they need to go to Ed Greenwood or Keith Baker? No. They own the IP, straight up. They can do whatever they want with it. Same for Dragonlance, Ravenloft, Dark Sun, and every other campaign setting outside the stuff like Star Wars or Wheel of Time that they licensed back in the 3rd Ed era.
What they're doing by bringing back the original creators to work on these settings - even if it's just in a consultation role - is getting validation for the new system. "You didn't like what we did in 4e? I'm sorry, we learned our lesson. Ed Greenwood himself is writing the new novel to fix it. We're also doing this huge open playtest to make sure we get the core system much closer to D&D's original roots. And we're starting to sell all our old back catalog you used to have to spend a lot of time searching for then paying outrageous collectible prices to get. We cool now?"
My gut feeling is that we're going to see Forgotten Realms be the "Default" setting. They'll then release "Themed books", where each book is tied to a genre or campaign style rather than a specific world. Rather than getting a Ravenloft setting book, you get a Horror-themed book with all the rules, class builds, monsters, etc. you'd need to run a horror-themed campaign. Then you'd get a pirate one for sea-based adventures, an urban one, a magitek one, a desert one, a political one, etc. This gives you all the rules you need without branding the rules to a specific setting, avoiding the pitfall of branding issues.
Then they'll release campaign books that are rules-free (similar to the Elminster's Forgotten Realms and Menzoberranzan books from last year) with guides for what additional rulebooks you'd need for each setting. So rather than cramming a bunch of rules in the Ravenloft book, it instead just talks about each of the different demiplanes. This opens up page count to more narrative-based setting books, which allows cross-platform sales so people who play Pathfinder or an OSR system can still purchase the campaign setting and use it in their system of choice. Want the specific D&D Next stats for Strahd? Check out Dragon Magazine #whatever (and expect those to switch from a subscription-only model to also allow purchase through the online store of individual issues).
I've got absolutely no solid evidence this is what they're going to do other than my gut feeling and extrapolating from the various interviews and Q&As going on. I don't even think WotC knows how they're going to do it yet since they're still focused on the rules right now. Branding and release schedules will probably get nailed down toward the end of the year or beginning of next year.