I'm not terribly familiar with GH, but it seems too familiar to the stuff we've already got to seem like a big selling supplement without seeming kind of stale if it's the 2010 release.
I voted Dark Sun because it seems most interesting to me - always has even if I didn't like certain aspects about how TSR handled it.
PHB2 adds in Primal, but really rounds out classes from 3.5 (Druid, Barbarian, Bard, Sorc)... Eberron seems to cry out to Psionics as does Dark Sun so I'm guessing that's the next power source. and would make a nice 2010 Dark Sun w/ PHB3 being perhaps Psionic. If we're filled out with the other sources, my guess would be Elemental as a secondary source to fit in well with Dark Sun's Elemental Clerics and overall elemental theme.
Ravenloft would work well near the release of the Shadow source. Perhaps the Ki source would best fit a new 4E-specific setting unless one of the above has a good region or theme that would make it ideal.
Maybe I'm completely wrong for thinking there will be a strong link to the order of settings and release schedule of new power sources, but if I had to put my money I'd guess they'll be closely connected.