I really depends on who you want to tailor the event to. I mean, some people like to be active just to talk about site plot developing, others like the event games, others like posting to get the exclusive event items/currency...others like to sell/make/get event art. Who knows.
If I recall, people have tried to run user-based events, and it flops. If I am wrong, then I want to be corrected. When those things do happen, they aren't advertised well and no one knows about it until it's already under-way.