I'd love to play a cross server instanced PvP dungeon, a lot like Darkness Falls in DAoC. Darkness Falls monsters yielded marks for unique gear and the dungeon bosses dropped moderately good PvE loot. It had three entrances for the different factions that progressed pretty linearly to the bottom, allowing for awesome small group and solo PvP action.

Stillmoor and Shimmersand are great PvP areas, but an open world and confined dungeon are different flavor world PvP areas.