I probably won't be playing any more MMOs until some developer realizes some very important things and makes a game that:
1. Doesn't invest money into creating some generic Good Vs. Evil or Faction vs. Faction or other generic storyline. Instead, developers should realize that the players can create more interesting stories than any wall of text questline, because it feels infinitely more real and important to the player.
For example, if a game turned on player vs. player combat and full loot across the world, your environment and playstyle becomes immediately more important.
Let's say you walk into a dungeon, but the monsters inside are too tough for you. You meet up with another player just outside the dungeon and team up with them. You fight your way together all the way to the bottom, defeat a big bad boss, and open a treasure chest. Inside is a +5 sword of awesome that you both want. You disagree on who gets to keep it, and one player hits the other and a fight to the death begins for the sword. This is just one example.
2. Recognize the importance of new content and plenty of endgame for the players. This can be achieved through procedurally generated content, randomized events, long-term goals of actual relevance, or even through releasing a game toolset to the players and allowing them to generate content that might make it into the game.
Neverwinter is trying this last one, but we have yet to see how it will be adapted and whether their approach will work out.
3. Innovating combat systems. I'm talking about moving past things like hotkey mashing games like WoW where half your screen is filled up by hotkeys, many of which you may not need to use for hours or days of gameplay. Furthermore, games like Guild Wars 2 would have been substantially more fun if they had simply dropped the targeting system entirely.
Imagine a melee combat system like the one used in Mount and Blade, except expanded beyond the 4 direction combat. Or imagine drawing symbols in the air with your mouse in order to cast spells. Really, there's plenty of room for imagination in gameplay, something that would have benefited the endless WoW clones.
