A Dungeon Keeper MMO
We all know how fast MMO content gets stale, forcing developers to produce more and more content to keep the player-base paying happy. World of Warcraft experiences a big loss in active players in the time before a big new chunk of content is introduced, simply because those players feel like there’s nothing interesting left to do anymore. PvP games don’t have this issue, or at least a lot less of it than PvE ones. As long as a PvP game is done well, players themselves will provide new content simply by trying to figure out new ways to beat their opponents. I’m looking for a way to introduce the variety that (good) PvP offers into MMO PvE gameplay. One idea I have is to allow players/guilds to build their own dungeons for others to fight in.
Players in this hypothetical MMO would have a hero character as is usual these days, but they would also be able to build a dungeon and improve it as their character progresses. One would need some kind of incentive for doing so as well as for trying to beat such a dungeon of course. Adventurers in the dungeon could be rewarded with items and experience depending on the strength of the dungeon while owners of a dungeon could be rewarded for defending the dungeon against those adventurers. This way, players would constantly try to make their dungeon hero-proof while at the same time figuring out new strategies to beat the contraptions that other players could come up with.

Imagine if you could have real players roaming your Dungeon Keeper dungeons instead of stupid NPCs.
Successful players/guilds would own very famous dungeons that players flock to from all over the world to beat, while less dedicated players would fill the world with a never-ending stream of easier dungeons to level up in. Instead of finding a tame tentacle and putting it into your guild house where no one ever sees it, you could have it greet adventurers at the entrance of your dungeon.
Building your dungeon could be done in a variety of ways, but I’m imagining it being done in the same view that normal adventuring happens. You would walk through your tunnels (or dig them first) and place objects and monsters right there in the 3D world. There are many ways that these could be acquired, of course. Hunters might capture rare monsters for your dungeon while engineers could come up with clever traps and dwarfs would (obviously) make sure that your stonework is solid. An individual dungeon owner might only be able to capture weaker monsters and would have to enlist the help of an engineer to even make the doors work, thereby creating a dungeon suited for single player adventures. A whole guild working together could be able to capture very tough boss monsters which would then provide quite the challenge for even larger groups of adventurers.
A problem with this idea is obviously balance and keeping players from exploiting the system. You need to make sure that players can’t somehow make a super easy dungeon with high level rewards for their friends (or other characters) to just farm rewards in. The other way around – dungeons abusing mechanics to be as obnoxious to players as possible – isn’t as much of a problem because players could simply avoid visiting such a dungeon in the first place. The whole system needs to be designed in such a way that dungeon owners want a lot of players to enter their dungeon but as few as possible to leave it alive.
I think if done well this could be a hell of a lot of fun for dungeon owners while providing a constant flow of interesting content for adventurers and reduce the content generation workload for developers to boot. What’s not to love?