I'd love to see an original 4e PoL setting that is different than most D&D settings- something dark, burtal, and desperate. Something very much in line with a S&S theme (FR has high fantasy down, and Eberron has wahoo fantasy).
That said, I'm prepared to be disappointed and see Dragonlance get the go-ahead from WotC. Dark Sun would surprise me simply because while it was a cool setting and beloved among hardcore gamers, it never had that huge of a following, and wouldn't probably have as much appeal as a more "traditional" setting.