It would work more or less like this:
- Players submit their entries when we're running a contest.
- We pick our favorite when it's over.
- We contact the winner and discuss the details, such as balancing the rewards, adjusting the requirements, etc.
- Once it's done, the achievement/mission is added in-game, the winner credited somewhere and awarded with something.
To avoid dealing with half-assed entries such as "kill x mobs, receive z exp", players are asked to put some creativity into it and state the purpose of their entry.
Purposes such as: make players revisit old maps and dungeons, promote doing quests, make a certain item available, create a hard challenge or simply add something that could be fun, etc.
Thoughts, ideas?