At the speed that MMO players churn through game content, game devs have no chance of keeping up with demand. The answer is simple, let the gamers assist with adding new content to the game. If adding game content was treated the same way as open source programming, then it would be a simple matter having one (or more) quality control game dev experts that regulate the contribution from gamers. Obviously not all gamer contributed content would be added to the game, just the stuff that was deemed acceptable by the game company.
If gamers contributed their own content to the MMO, then other gamers would be happy for the extra content and game companies would be happy for the cheap labour and gamer retention. I only see a win-win scenario. The only thing that the games companies would need to flip the cost for are the editors required to produce the extra content. Technically the editors are created before the game anyway, they would just need to by made 'user friendly'.
Player created content is not a new concept, it has been around for decades. Me thinks back to my Command and Conquer level editor. As games have become more complex, so to has the tools to create them, and the thus the reduction in editors for creating content. One solution would be to make very specific editors that only target specific areas of the game. For example, a character model editor for WoW, that would allow you to submit different hair styles, faces, bodies and extras. This would enhance gamer experience through greater customization of their characters. Some other examples would be weapon editors, clothing editors, quest writing editors, combat animation editors, PvP map editors etc.
Pirates of the Burning Sea provided the ability to submit your own ship, flag and mask designs. What a seriously great idea! I could only imagine how happy the gamer felt when he saw his ship design being sailed around by 5% of the online population. What an awesome feeling of achievement! I just wish I could submit my own cape design to the devs at ArenaNet so that I could have it on my Guild Wars characters. It would certainly make my guild that extra bit cooler. I remember back to the days of Never Winter Nights when I would often play on the gamer created persistent worlds. A lot of the time, they were better than the actual game.
I just hope that in the future MMOG companies make it easier for players to create their own content, because it would make for one hell of a better game.