There is nothing special about the idea of an Elder Scrolls MMO. The Elder Scrolls world is just another western fantasy setting that has virtually nothing to distinguish itself from D&D, Tolkien, Warhammer, Kingdoms of Amalur, Dragon Age, or Warcraft. The only thing that would be different would be the company that makes it, and there is no evidence that Bethesda can do a strictly "better" job than Blizzard. The difference between single player Bethesda games and MMOs in general (a genre that has seen almost no mechanical changes since Everquest anyway) is really only the control system. I suspect the controls like those of the Bethesda games haven't yet been explored by other MMOs (to my knowledge) is probably because of connection issues. Imagine how pissed players will be if they seem from their own perspective to dodge or hit enemies in Skyrim gameplay, but the game doesn't read it that way and they get hurt or die in the flawed exchange. These things still happen in FPS multiplayer games. Imagine the extent of this problem on an MMO server. I'm afraid they might conform to the die-roll chance to hit format, or worse, automated dps with sword swinging animation, in order to avoid this problem. In this case, how would an Elder Scrolls MMO be different from WOW, DAoC, or Everquest?
Having said that, if the rumors are true and Bethesda is going the "prequel" route to the "plot," they would at least be leaving what little originality there is in the Elder Scrolls world virtually unharmed whether it's a success or not. They can leave the future from Skyrim available to expand on for ES VI, and there's enough time between the first game and the "prequel" that the actions of all these MMO players won't really matter to the story developed thus far. I won't be playing this MMO if it is really going to exist, and even if I did, I don't really care about the Elder Scrolls world to begin with. I just like to explore pretty forests and have thrilling, deeply immersive adventures. I think leet speak would take me out of that experience.