Something I've been thinking about that could fit with this. Basically the introduction of Legendary Mobs.
At any time there could be a chance for a Legendary to spawn anywhere in the game world. Only one Legendary could be in existence at any time.Legendary would use a model chosen from those already ingame, ie Player character, npc, or creature. Legendary would have a random generated unique name and have sufficient ability and stat level to be seriously difficult for a group of high stat/ability characters to overcome.
Legendary could have a patrol path likely to bring it into contact with other mobs, npc's and players.
I would suggest behaviour to be either aggressive or passive, determined on creation.
Aggressive would attack anything it encountered whether they be player ,npc, or mob. [They could have the ability to kill quest npc's for instance encouraging people to group and deal with it].
Passive would defend itself, otherwise wander the landscape. [Offers multiple rp opportunities. Would one group be in favour of protecting this being while another wants to get rid of it?]
Unsure about loot, PP, rewards in general. Perhaps dealing with the threat alone would be sufficient.