In my opinion, I think that the best possible MMO that can be made out of Star Wars would be a non-canon to the storyline one. Imagine just the universe, and you would be some kind of freelancer in it, and have a job, like bounty hunter, entertainer, diplomat, Czerka Corporation employee and such, and not being necessarily Jedi or Sith, part of the Republic or the Sith Empire, just, as Jango Fett says in the movies, "I'm just a simple man trying to make my way in the universe.", or something like that. The idea of just Sith and Jedi fighting in a MMO sounds pretty dumb and uninteresting to me - I hope there'll be professions and such, and freelance exploring and doing interesting story-driven quests. I hope it'll be like that, not just simple killing.
But, the possibility of joining a Jedi/Sith academy and learning the ways of the Force shouldn't be left out, either. But being Jedi/Sith shouldn't have an effect on the jobs you do to make your living. As far as I could tell by reading the description of the SW MMO, it states that "choose to be a Jedi or Sith". I don't think there'll be much variety in it.
And also, the events in the galaxy that would be determined by other players, should have an effect on the galaxy itself. Just like in KotOR, poisoning the kolto -> the price for the medpacks raises. And, during the Jedi/Sith war, there would be different things to do in a specific planet determined if it is either controlled by the Sith or the Jedi. Important thing that it shouldn't be left out is the fact that there should be different organisations than the Sith and the Jedi - like the Exchange and Pirate organisations, such as the Hutts.