Many a single-player game comes with some sort of map editor, and the ability to load and play on maps created by other players. Of course not all of those maps are good, but then there are often websites from which you can download these maps, and rate them, and through the ratings of other players find the good ones. This player-created content adds a lot to the longevity of the game. So I wonder if something similar would be possible for a classic MMORPG like World of Warcraft.
Now virtual worlds like Second Life are all about player-created content, but there is no game behind them. Sure, you can design a fancy shirt or a curiously shaped sofa, but besides looking good these items don't do very much. And there is a danger that player's creations tend towards the obscene or try to promote real world ideas or brands. In a classic fantasy MMORPG you wouldn't be able to create just about anything, because that could break the game. But if you have reasonable restrictions, why shouldn't players be allowed to create for example a dungeon?
Games like Anarchy Online or City of Heroes have random dungeons put together from tile sets and populated with random monsters corresponding to that tile set. Player-created dungeons could use similar tile sets, with a dungeon editor to put them together. You choose a tile set and a level, then build the dungeon by linking together tiles of corridors and rooms and stairs. You get a selection of mobs that fit with the level and tile set, and you can place them in your dungeon, and determine patrol paths or spawn points. There could be a couple of boss mobs to place, with different special abilities. There could even be some sort of event editor, adding some special effects, doors opening only when a boss mob is killed, or events like ambushes to the dungeon. You would not be able to place rewards yourself, because you shouldn't be able to make a dungeon full of treasure chests and no monsters for your friends to harvest. Instead the rewards are placed automatically using standard loot tables for the monsters you placed.
For the players in the game, there would be special entrances to these player-created dungeons in zones of the corresponding level. Once your group enters the instance portal, the group leader is presented with a list of all player-created dungeons for this level, which can be sorted by popularity rating, size, creation date, or the name of the player who made them. When the group leaves the dungeon, regardless whether they finished it or just gave up, they will have the opportunity to rate the dungeon on a scale of 1 to 10, thus creating the popularity rating mentioned before. But every player can rate every dungeon only once.
Now I'm not saying that this would result in dungeons as good as the existing developer-created instances. But the best of the player-created dungeons could come close. And obviously there would be a lot more of them. Would you rather run through the Deadmines again and again, or try a player-created dungeon of the same level?