Essentially, the idea is that in the last week of each month, a poll is posted containing several rare items that are in high demand - a time capsule, a kaio-ken capsule, what have you. Users would vote on this.
When the next month rolls around, a tournament begins - some number of people would enter, perhaps for a small fee (ex. 2,500 zeni), and it would be a standard tournament - and the winner would get the rare item.
Perhaps make the tournament once every two months instead of each month, but such details can be worked out later.
EDIT: Well, the reward doesn't necessarily have to be an item from the shop, it could be some special item or what have you (ex. faction tickets, unique "artifacts" or other rewards, etc.)