The wave of mass market MMORPGs that emerged over the past years have mostly been emulations of the World of Warcraft formula. Perhaps the biggest "innovation" we've witnessed since WoW launched in 2004 has been the gravitation towards free-to-play.
The investment needed to create an MMORPG on the same scale is so high that naturally publishers and developers are averse to taking risks, therefore reducing the potential for innovation.
The reason so many blockbuster MMO games have struggled has been due to a lack of sustainable content. Players need sustainable content and engaging combat systems. Without sustainable content, hardcore players will always be left unsatisfied and casual players will get tired of the raiding "treadmill". There are only so many times you can raid an instance or defeat a boss.
Blizzard did an OK job on sustainable content with World of Warcraft. Combat mechanics had enough depth to engage people for a sustained time and there was significant enough skill-curve for even the veteran players to learn new things. Battlegrounds and Arena are good examples of engaging sustainable content. Essentially the underlying game mechanics were well balanced enough for the PvP to have depth, but even the longevity of PvP was sometimes driven by the need to farm items.
Where Blizzard started to go wrong, in my view, was around late 2007 when the capacity for deep gameplay was reduced in favor of appealing to those outside of its core audience. WoW had become pop culture by this point, and Blizzard responded by trying to appeal to the masses. This in turn diluted the overall feel and potential of the game. A good example is how the designers made an array of active abilities passive and how they "streamlined" the class skill trees. What it all boiled down to was reducing the diversity of the gameplay and the choices that players could make. By attempting to appease everyone, Blizzard pleased no one. Subscriber numbers were rising due to marketing spend, but the core World of Warcraft community has been in decline for some time.
The future lies with sustainable content and user generated content. Gamers love to feel a sense of ownership in MMO games. Having the opportunity to create content has far reaching benefits. User generated content will increase the sense of value players place on the time they invest into the game, and when they have a sense of ownership it will deepen their relationship with the game. There is a reason why most people spend a long time on the character creation screen.
With World of Warcraft, players eventually realize that "nothing" truly belongs to them. As soon as you've defeated one boss or completed one quest, you move onto the next one. Unless you are among the top 0.1% of users, you will never be the first to achieve anything. Everything will start to feel recycled. Even if you are in that top 0.1%, there will always be a new instance on the horizon. Everything you've worked so hard for in the game will eventually be devalued when the next update or content patch arrives. The game is essentially a treadmill in this respect.
The lack of consequences to your choices will also be a key factor in WoW's decline. A purely cosmetic example of this is how you can now pay to change your character's face, haircut or appearance at any time after character creation. Little things like this can make an MMORPG feel disposable. There are too few possibilities to influence or affect real change within the game world.
What players really crave is an experience that gives them something to show for their efforts other than predefined loot. An experience where their personality, creativity, teamwork or resilience is somehow reflected and etched into the game.
The MMORPG to overtake World of Warcraft will put more emphasis on player driven content which will, in turn, fuel creative and emergent gameplay. Nothing will feel static because the world will be constantly reacting to the actions of the communities that shape it. It will probably be free-to-play but will approach monetization in a way that isn't detrimental to the game. It may have the capacity to be played competitively. It will have deep combat, building or skill systems but will also be accessible to new players.