It depends on the nature of your thread. If you have written a story, wish to have other people join in writing a story, or a roleplay about a war between bosses then the thread would go to the Forum Games & Stories sub-forum.
If you plan on making a poll of some sort, 'cleverly' titled "Boss Wars" asking people to vote on their favorite boss, or who they believe would win in some sort of grandiose battle, that thread would belong either in Mario Chat (If it's only for Mario Bosses) or Video Game Chat.
I appreciate your taking the time to ask before posting, and in the future, please feel free to use the Private Message system to contact a Moderator or Administrator about any further questions of this kind. Thanks, and have a good time posting!