Wrath of the Lich King
A "cool part of Northrend is new big PvP zone. It's about the size of Westfall, and will be all-PvP - in no way, shape or form will we support PvE players in it", the developer's Jeff Kaplan has told PC Zone.
He continued, "It'll be optional, but it'll be completely non-instanced, have multiple objectives, siege weapons and destructible buildings".