These days, when a company sets out to create a Massively Multiplayer Online Role-Playing Game, they don't so much of think of innovative new ideas. Rather they take other features from previous MMOs that players like, tweak them a little to either make them better or their own, and implement them. So naturally, to make the perfect MMO, it is only a case finding the best features to steal.
MMO design boils down to several categories that you need to fill, which include: story, character creation and customization, boss fights, quest design, loot , battle system, PvP, and crafting systems. Now it is only a matter of plucking the finest from the MMOs already out there.