Even now I think all these games are fairly formulaic with the same basic mechanics and psychological devices (compulsion loops, etc).
Basically, MMOGs are all like resort casinos. There's some spectacle and entertainment. The driving mechanic is the addictive variable schedule of reward.
I'd like to make a game where true exploration is the base mechanic. There would be no storyline, just exploration. All content would be either procedurally generated, created by the users, or evolved through genetic algorithms.