Does your fandom not have a forum yet? This is the place to fix that!
Just post here requesting the fandom; admins will then decide whether it merits a forum, or would be better served by a discussion thread or forum elsewhere on the site.
Please note that if you are looking for a forum for your own work this is probably not the appropriate place. Unless you are an internationally famous writer or something, in which case we humbly grovel at your feet and beg forgiveness.