I had a similar thought, my guess is that the "next generation" of MMOs has humans playing the 'Boss' in dungeon scenarios. This makes re-playing the content interesting (the talent of the boss can change) and off sets a fairly large technical burden (scripted encounters). Of course it adds issues with potential collusion, balancing difficulty, and continuity (you need someone online as the 'boss' for the encounter to happen).
I sketched out an update for WoW that would allow RBC's (remote Boss characters) to be statues in game, until someone logged in and "inhabited them" where upon there would be an in-game effect (like 20 - 30 seconds of a glowing around the statue) and then 'wham' the boss comes alive. Until either dead, or they log off and the return to being a statue.
Its a complex mechanic but it has the potential of providing a lot of quality game play for the subscribers. I'd need an economist though to get through the models of expected subscriber return value vs cost.