Fantasy has always seemed to dominate the MMO market. To the rest of the gaming world fantasy is just a niche. Each MMO seems to just be another copy of the original idea, another version of a hunter, assassin, mage.
There have been attempts to introduce new themes to a MMO but most aren't very successful. Are people just going to keep playing fantasy, or are we going to see any diversification?
The common favourite to take fantasies place seems to be sci-fi, some people think that if you take a genre that works well for another game type such as FPS and merge that with a MMO you will get the best of both worlds. I'm not so sure when people seem content to just play the same type of game over and over.
It doesn't seem to set to change, all the games I have heard of being currently in development are just more fantasy rehashes, the only exceptions are Blizzards new MMO and Planetside 2. And with both of those we have no idea when any more information will become available.