Here is the thing, I don't think there is a lot of room to do a setting that is generic enough to fit the idea of the classic D&D stereotype without basically ending up with a retread or pastiche of a setting that already exist, like Greyhawk, Forgotten Realms, Mystara, et c. It wouldn't be worth the effort IMHO to make something new when an update to an existing setting would likely sell better anyway. As for tying those too closely together in one setting, well a lot of people don't take kindly to an extensive level of tinkering with classic setting, it would cause a crapstorm. To make something new that would be worth the time it would have be at least different enough to take it away from that ultra generic state that it wouldn't be a proper fit for everyone's D&D game, at least as different as Eberron if not more so, so while I expect they will make a new setting it may be a bit before they get around to it.
Of course I could be wrong, people have been eating up Golarion like nobody's business. But the flipside of that is a huge list of also rans, many of which came from WotC or TSR themselves, not to mention other companies.