I see a few possibilities.
Jotunheim, located in northwest Icecrown, the home of the new characters that storm through Icecrown in the Wrathgate cinematic. Not only would it progress the lore behind the characters, it'd offer quite a lovely Norse mythology flavor to the game.
Uldum, a decent follow up to Ulduar? Being both made by the titans, it might be too much titan stuff right at first, but it might come eventually?
Grim Batol, the Dwarven, then Orc stronghold in the Wetlands. Lots and lots of First and Second war lore involved there. Red dragons, orcs, dwarves, and most of all, it'll take us all back to our low 20's