Unfortunately, I think the chances of Mystara or Greyhawk coming back are fairly low, because of their similarity to FR. The same goes to a lesser extent for Dragonlance and maybe even Eberron (though Eberron has a fairly strong identity of its own).
There are very few things you can do with Mystara/GH that you can't do with FR. Sure, the countries are different, but they're all still fundamentally "basic D&D-style fantasy" settings. Planescape, Dark Sun, Spelljammer, Ravenloft, and Al-Qadim (just to take some of the stronger flavors) all have their own hooks: Planar travel + Sigil, Savage deserts/sword & sandals-style, SPACE!!!, Gothic horror, and 1001 nights-style Arabia.