What would the ultimate, unstoppable MMO look like? Well, it would need to be fully peer-to-peer, so that it could outlive the rise and fall of any given company, or at least free and easy to run a server. And anyone who runs a server (or the world on their personal machine) would need full power over what goes on there. It would need to incorporate gameplay that's both exploration and world-building, combat and defense, and all the best of elements that have made other games successful, preferably through add-on modifications so that it can incorporate any new features that come up, too. And finally it would need to have infinite possibilities for new fun, so that existing players don't burn out on the whole concept and go play something else. That means players need to be creating new content all the time, as part of the game.
Mokalus of Borg
PS - Actually, that sounds a lot like the internet itself.
PPS - Except that's not so much a game we're playing.