I'd love to see an MMO where the players can actually develop their own instances and build them directly into the game world. Environments and boss encounters developed by players, for players.
This would essentially be like building mods for an MMO, and would (theoretically):
a) Reduce the workload for the development team in creating new content,
b) Provide a greater variety of environments (there's only so many bloody Zul'[insert keyword] troll instances I can handle),
c) Allow direct player/designer feedback, and
d) Let the community create content with difficulty matching the target audience. The more "hardcore" guilds often complete endgame content before it's even reached the public test realms, yet your casual guilds will often miss out on a lot of the mid-level content. Blizzard's inclusion of the heroic raid option in Lich King was a great idea, letting casual guilds enjoy semi-endgame content, with aspirations of moving to a higher difficulty.