MMORPG's are good, but the player never has any control over the history of the world. They always do quests for NPCs and gain EXP, GP, Items what have you, but they never can completely change the history of the world. It'd be difficult, I myself am not sure if it would even work, but I think a good idea would be events that happen once and only once to one player or one party. Of course, a large team (I'm talking a good 200 people) would be constantly creating new quests and things for players to do. Once they're done, they're done for good. No other players can experience the quest, just like real life. But finding someones lost dog isn't gonna change the world, but every so often the game could have wars between factions each player is in, or other things of earth shattering proportion. But what about new players who don't know the history? Libraries would be updated often with history books for new players to read and get the jist of the past events in the MMORPG world. Impossible, mayhaps. Awsome, yes.

Unfortunately, I think you realise the problem here - that much content just can't be generated that quickly. Nor is it likely to be of a consistently high quality. And neither is it likely to be appreciated since much of the content will end up feeling the same anyway, even if it is theoretically unique. The nearest you're likely to get is some sort of procedurally-generated content.