Corse its going to be in an expansion, its just Northrend. Its not like Azeroth or Kalimdor, which are both larger than it. It'll probably be added after the South Seas islands are added, considering they're pretty interesting
Since, you know, they have goblins and all. All Northrend has are Nerubians (Yay!), blue dragons, furbolgs, tuskarr, a jungle, a crystal forest, a LOT of snow, Storm giants, Magnataur and some other stuff. The South Sea islands would have terrain that isn't perminently coated in snow, it'd have varying terrain depending on the island you're on. And underwater instances, which is something people have wanted for a while. Northrend has about... 4 instances of worth: Old/New Kingdoms of Azjol'Nerub, Icecrown Citidel, and Zul'Drakk. Oh, and the South Seas are where Queen Azshara just happens to be located
... That, and I want a pet Coutal
Oh, and then you have Arthas and his army being all on Icecrown Glacier and no where else. And that of corse will be high level because Arthas would rip through level 60s like paper
Lorewise, who's gonna be dumb enough to sail up to Northrend anyway? They never pretended it wasn't there, its just that no one goes there because there's enough strife happening around where they live