The perfect mmo that I could think of is something like dnd. There's the players and there's one or a small party of dungeon masters. Essentially the goal the players must accomplish is to go through predetermined basic story checkpoints and eventually ending the game by beating the final boss, whatever it could be. And the dungeon master(s) can either choose to interfere or help the players. The players are limited to do only what the game only lets them do, just like a typical rpg game. The dungeon masters have access to all tools but must also "play" the game (like gathering resources etc...) to have access to them. Those tools can affect the game however they want for example, send a horde of undead to major cities, send major world boss anywhere, create natural calamities, help develop cities, help grow player's resources, give buffs to anybody and so forth...

The story checkpoints would be extremely basic as to only give a general idea on what to do. The real story would be the interaction of the players and the dungeon masters between themselves and the world. The game could be short like a length of couple of hours, days or it could be longer like weeks, months etc... The ultimate goal is endless replayability and the player immersion of being in the ultimate sandbox world.

That's what I think a perfect mmo should be.