What we want, and what we think might happen need not to be the same thing. That said, I do wish they would make PoLands into a real setting. But I am fairly easy to please, so unless they make Spelljammer or Dragonlance, I think I might be tempted to buy it.
I second (or third?) that they will hopefully use one of the ones from the campaign search that netted Eberron. I believe they bought the rights for all the finalists (correct?) and Eberron is fantastic, I hope that one of the others were just under Eberron in awesome-ness to develop.
I think 4e deserves it own setting. Upgrading existing setting is all well and fun, but I'm itching for something new, something I can open the cover and read page after page of wonder.
I also wouldn't mind a shift in setting period. I realize Dark Sun does this going back in time, and Eberron does a good bit of this going forward, but going forward even more into the modern realm where certain technological (or technomagical) discoveries have been made, would be interesting.
There is also a lot that can be borrowed from sci-fi, gate travel between worlds, parallel universes, techno gods, etc.
I'm not really looking for originality, that's a tough order, but I'd love to see something new that's at least a little unorthodox.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.