I would love a MMORPG game that had sufficient story arcs without feeling like the effort is meaningless. For example, a game situated in a constant universe where the entire player base became an alien race trying to survive on their planet. Time would of course have to be faster than 1:1, so players would inhabit lineages of family units, and new players would be 'born' into their own lineages branching off of already established ones. Actual extinction events would wipe out the civilization, and so efforts could be made to leave caches in the stars or in orbit of small moons for future civilizations (probably the same players) to go retrieve. The biggest issue I can see with this concept is that most of the content would either have to be generated by the creator (e.g. something bigger than an extinction level event), or arbitrarily by the participants.