It's one thing to say that you want to tell a good developer generated story, it's another thing to explain how you are going to tell a good story in a true MMO given the limitations of the medium.

The basic problem being that if you allow the world to change based on player actions, then only a tiny percentage of hardcore players will ever see your story (everyone else will only see the after effects).

But if you don't allow the world to change, then everyone gets to see the story, but suspension of disbelief gets stretched to its breaking point which ruins emotional involvement in the story (players are told that their actions have an effect, but when the players return to an area, nothing has changed).

The usual way that game designers get around this problem is to instance portions of the content (or "phasing" which is just a variation on instancing). But, as I was alluding to above by saying "true MMO", the more you instance, the less your game is really an MMO.