I voted for Eberron and Greyhawk. I think Greyhawk is probably best left retired, but I still think it's leaps above the Realms (the success of which still boggles my mind).
Really, I'll be pretty disappointed if the next setting isn't Eberron. It was pretty much the high-water mark for 3E and has a large fan base -- larger than Dark Sun and still very inclusive of psionics.
After that, I wouldn't say "no" to Ravenloft, Dark Sun, or Dragonlance. I'd be fine with a new setting, too.
What I'd like to see is two adventure paths every year. One would be generic (preferable) or loosely coupled to the Realms (I don't care for it, but there's demand). The other would be tied to a specific campaign setting and be released in conjunction with that setting book (give or take a month). One rules supplement would be released, each year. Don't force it, but try to include some tie to the setting. Psionics goes well with Eberron or Dark Sun. Dragon and/or magic might go with Dragonlance, Council of Wyrms, or Planescape. An Unearthed Arcana/PHB2, with lots of new sub-classes and feats might go well with Dark Sun or Ravenloft. A Monster Manual 2 could go with just about any setting, but best with Planescape. Again, don't force it, just a minor tie-in. I'm really thinking of the same sort of books as 1E AD&D or 2E Options.
Even better, but it doesn't sound like its in the cards, would be to only release one AP per year, tied to the setting. Resurrect Dungeon for the other AP and for general adventures. Only release a rules book if there's obviously enough material to fill in (Psionics is the only thing that comes to mind, but MM2 is certainly possible). Put campaign-centric rules (artificer, warforged) in the campaign setting book and use Dragon for general stuff (genasi, changeling, feats).
Really, I'd like to see the hard-covers reserved for stuff that's done in-house, or with heavy in-house oversight. Use Dragon/Dungeon for the farming out.