Secret of Mana had a wonderful feature that allowed you complete control over 3 characters at once. You could just control 1 character and allow the AI to operate the others. You could control all three in an alternating pattern. Or you could even bring a friend along and allow them to control the second and third characters for you. Additionally, you or your friend could enter and exit the game at any time, as long as one player was always present.
Is this one of the secrets to the game's success? Let's look at another example...the highly popular LEGO game franchise. If you've played any of these, you'll notice a similar mechanic. In single player mode, an A.I. controls your companion. But at any moment, a friend can join in, or leave the game.
This leads me to a suggestion for MMO's. Suppose you allow players to share a character across accounts. Now, I could have multiple characters, and if my friend wants to raid with me, I can share that character with him.
Suppose I'm in a raid, and I have to go elsewhere for awhile...I could leave and let AI take over my character for awhile, so long as another human player is still around.
Suppose we need to fill more slots in a raid...What if I could bring 2 of my other characters along and allow them to AI battle for me, or swap between the three at any moment?
It's all about more power to the players, and more choices of how to play. I'd like to see someone try this out.