A zone where you could join a faction and then go out and fight PvE mobs for loot you have to carry back to base but can be jumped on by another faction trying to steal your goods would be really fun. Or capturing and holding forts like in DAOC or GW2's WvW mode or WoW's Wintergrasp/Alterac Valley maps. Though as Archeron said, Square would need to become serious about PvP.