Spheres of Advancement: I got this idea more from Vanguard : SOH, with several different spheres, (Adventuring, Crafting, and Diplomacy, Harvesting used to be a sphere as well) and I think Next Gen MMO's need to capitalize on this concept, because these massive worlds can appeal to a large base of people without taking away from the core adventurers by adding in other sphere concepts that they can level up.
This could be where alot of solo content is added as well. Examples would be a game with Aventuring sphere, pvp sphere (different sets of gear and skills from adventuring, balanced for pvp action.), Crafting, Racing, Entertainment, hunting, fishing, building, diplomat, etc
All these spheres would have its own set of gear, and ways of advancing it, and not necessarily dependent on any other sphere. So lets say you aren't much of an adventuring player, but would love a game where you can raise horses and train them for speed, jumping, or stability, you can then sell your mount to other players, or you can race them in a track against other players, possibly winning trophies or prizes.
The more spheres and diversity in the game, the larger crowd of people you can appeal too without pissing off your other players who like certain spheres certain ways. I believe this concept is what is going to make or break future mmo's.
A lot of games already have this concept somewhat, but I think each thing needs to be defined more as its own separate sphere, allowing for quests, gear, levels or skill points.