Consider Tobold's proposal and read my recent comment about Sacrifice:There are five main paths to the end goal one offered by each of five Gods. Each God offers a sequence of nine missions leading to a common tenth "final mission". Each God has their own style and rewards the player with unique powers and creatures for taking their missions. The really clever bit is that you can mix and match missions for different Gods leading to a wide range of different paths through the game and resulting character builds. Some combinations are prohibited because a God you have fought against too often will refuse to offer you a new mission. The number of permutations is still large. Amazingly the game manages to tell a clever story which hangs together despite the random selection of paths to get to the end.
Putting the two together I think it might be possible to use a variant of Sacrifices multi-path system in a massively multiplayer format. The power of combinatorials gives a very wide range of unique quest paths from a finite number of quests. It would be particularly interesting to implement Sacrifice's skill model where the skills you end up with depend on the path you take through the game. Of course end game focussed players and min-maxers would soon figure out the paths which give the best build for healing or tanking or whatever but I imagine many other players would be happy to enjoy the journey.
PS. Yes I know mmos are history and I know it is pointless talking about them but this isn't an old style mmo its a proposal for a new style mmo so its all right to talk about, so there!