I came up with an idea the other day that I think can best be highlighted with the following scenario:
So, you're rooting around in some dark tunnels and come across some powerful necromancers who offer to teach you necromancy. So, you accept, and this puts you on a special new path. At this point your KOS to all holy clerics, and paladins who may be guards, merchants, etc. You gain some power cool dots, and a new spell- raise the dead. At this point you have a quest to raise an undead army. But- here's the kicker - you can attack other players, and they can attack you. You become a PvP BOSS in the sandbox. Other players will know, on sight, that you are an enemy and that they can kill you and will be rewarded for doing so. You're name will be changed temporarily as will your appearance.
As you kill things and pillage graveyards for corpses you can summon more and more undead, each of which will assist you in fighting players and amassing power. You cannot loot creatures anymore and you don't get experience for killing NPCs- you can only get experience from completing your new necro tasks- raising x number of dead into an army and killing a player town and all of it's citizens (including players). When you do this it will act as a rallying cry for players to work together to take you down! Even after completing this task you can continue to gain experience for holding the town and can turn this town into your personal base. However, once killed you will loose your spells, your army, etc, but you will get to retain the experience that you gained.