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MMO theme park quests too boring or simple? Suicide missions might be interesting.
Premise: Impossible missions/quests
Another presmise to this general concept, might be that there the quest itself is unknowable, never knowing when it ends, or what will happen.
The gist of this concept, is for developing quests that are so hard in some way, that there is no way you can realistically succeed. The things that players get out of this, is having an adventure, as if moving though a quest maze, and still get to achieve things along the way.
Repeating such a "quest", would probably infer some familiarity with the whole thing.
Even better, to change the quest structure every day/week/month.
Pffft, end up fighting dragons? You are likely dead.
Even better, additional loot you find are the corpses of other players that died venturing an area or such a quest in greneral.