A monster so big that it's in 3 locations and you have to kill all 3 "parts" 20 seconds from the others or it will heal. Some mage who opened a portal to a demon realm and you have to stop him. Some players have to kill demon spawns and some will have to collect pieces of ancient crystal t...

Just a random idea that banged me on the head. For every domain that the game is based on a major foe can be created. Some humongous dragon, giant or whoever. In some cases a big event in some cases just a common fight that will be fun, in most cases a "something more than a solo" thing. Y...