Traditional AAA mmos will never be "successful", if you define "success" as being popular.
Even financially, they pale in comparison to almost everything else.
What developer wants to spend millions to a billion on a AAA MMO when they could spend a million or less on a mobile game or the new hot shit (MOBA, BR, Auto Chess, etc.) and rake in so much more.
The next MMO that will be successful will be when Facebook becomes a virtual world.
I'm thinking like the movie : Ready Player One.
Social, VR, Massive Virtual World, etc.
Social is the key, the game mechanics won't even matter. Just make a dress-up game for your avatar and have all your facebook friends connected.
Very similar concept to Second Life, except a version 2 that is more mainstream with modern features + VR + Mobile