Have a look at all of the subjects in the Football section and see if you can apply any of these to the sport that you like.
None are off limits. Rugby, cricket, motor sport, ice hockey, field hockey, golf, netball, tennis, cycling, squash, swimming, martial arts, Olympic sports, etc.
Every sport has a story to tell. Are you the person to tell it? Have a look through the football list and see if you want to promote your story or sport.
Also look through the Autobiography - Biography section. Maybe the sport itself is not big enough to warrant a book, but an individual within it transcends the sport (Phil Taylor in darts, Ronnie O’Sullivan in snooker, or Tom Daley in diving).
See the Commissioning page for details on how to submit your idea.