I highly doubt this is for an Eberron box set. Isn't the new Eberron producting coming out this year, in a few months? They wouldn't be polling like this just five months in advance.
My money would be on Forgotten Realms. It just makes all kinds of sense.
Greyhawk is possible, but let's face it: the setting is a bit anachronistic for contemporary sensibilities. Duchy of Geoff? Nyr Dyv? Maybe a bit later as a legacy/collector's product, but for some reason it just seems more dated than the FR, perhaps because the creator was a pre-Boomer, actually "Silent Generation" (born 1938), while Ed Greenwood was late Boomer/fringe Gen X (born 1959). I just don't see them doing their first new setting box as Greyhawk (if that's what they're doing).
Planescape would be wonderful too.
Of course the best case scenario is "all of the above," but that seems unrealistic, or rather a bit optimistic. But I could see a smattering of box sets over the next two to three years: FR, Planescape, Greyhawk, maybe Dark Sun or even Mystara.