^^ Definitely a kind of "Dear Abby" type column. Simply a question and answer column on horses, health, showing, etc.
Also, I personally (since I am a beginner at this stuff), maybe have like, a "Horses 101" type column. You could talk about the proper way of handling a horse, overcoming common problems, grooming and showing tips etc.
Then, of course, something a bit more advanced for the advanced riders like... I dunno... something advanced. :p How to "fine tune" your riding or something like that.
Maybe a story column where readers can send in horse-related stories, poems etc. I know I like to read stuff like that.
Hahah, I seemed to have rambled a bit. Hope this helps a little!