Personally, I'd love nothing more than for them to release a Dark*Matter d20 Modern setting, with the whole setting updated, possibly reprints/updates of the Polyhedron materials, expanded updates/conversions of Dark*Matter creatures not printed in the Menace Manual, and so on, but I know it's unlikely.
On the same token, a revival of an older WotC setting like Dark Sun, Birthright or Mystara as a single-book publication (like Ghostwalk) could look like a whole new setting to a newer gamer who doesn't know the history of D&D.
It could be a "campaign model" mini setting like Shadow Slayers, Agents of Psi, Dimension Rangers, Age of Adventure, or anything else from the d20 Modern line that is going to get a few pages in d20 Cyberspace or d20 Apocalypse.
Generally, I also have trouble believing that they will make yet another completely new, fully supported D&D setting right after Eberron since that was known to be a big 2nd Edition fault. After all, look at how many people bash the Realms, or Eberron, or refuse to play in one setting or the other, and how many people only want to play that setting, or take their elements of that setting to other ones where it might not fit as well (like Warforged in non-Eberron games, or Harper Agents in non-Realms campaigns).
And as it was pointed out, it could be just someone trying to make waves and spread rumors. I don't know the contents of a standard WotC NDA, but even telling people you're playtesting a new campaign setting for WotC before it's announced in any way, shape or form is likely a breach on it's own. That sounds like the kind of things that NDA's are there to stop, especially in the modern age where one slip of the tongue from a playtester can go on the internet and become a worldwide industrywide gaming rumor in a few hours.