If you want to be a cynic, or you just happen to be one anyway, it's easy to complain that MMOs have turned into something of a cookie-cutter genre. So many of the basic expectations that players have are all but carved into stone, and there's a well-understood set of basics, ranging from quests to default control schemes. Of course, there's no shortage of variances between the biggest titles, but there's certainly a general idea in people's heads of what an MMO should look like by default.
Of course, any artist knows the best thing for when a genre starts to stagnate: trying something truly absurd, something that breaks down basic assumptions of what should be done. And we're seeing games with systems that do precisely that, like TERA's action-oriented combat with no lock-on or Guild Wars 2 experimenting with environmental gameplay. But those only scratch the surface of all the assumptions about MMOs that could be turned on their ear. What insane idea would you like to see a game put into practice?