Hah, I actually want to make an MMO or two someday...
My idea of what I would consider a perfect MMO would be as follows. First of all, there would be no levels or classes. Instead, the player could learn pretty much anything through time and effort, and develop their own class name or title. Second, the character would be able to age within game over a certain period of time, would have to eat right, get a part-time job between adventures, and possibly even if they were kids or teens, go to school on a regular basis.
The PCs could also establish relationships with NPCs up to a certain point, and if they wanted to, they could establish a small party with their main character and their alts, which would act upon an AI determined by choices made in the character generation process. You could also form relationships with your alts father than that of the NPCs. The alts of course, would have their own lives, jobs, goals and such, and the main would have to take those into consideration.
Randomly generated dungeons that vary in difficulty compared to both menu options and a player's general skills would also be available, but they would be accessible only by things like a portal, a pier, airship whatever...
In regards to the alts, they would function the same in aging, diet, and other things as well, and eventually, characters could retire after a certain point, or even reincarnate into a new character with a few of the best skills intact. Optionally, a PC and an alt could have an heir, and the heir could take over after a certain point, decidable by the player.
A housing system, along with many other features, like custom outfits, custom weapons, and even custom spells would be available too.
Finally, the entire storyline would be quintessentially roleplayed by the players themselves, though a background of the world would be available. There could be a different story going on in each server even.
These are just a few ideas I have for what I would call a perfect MMO. I have many, many more.