"Amateur" and "Pro" are opposites, but "Fan" and "Pro" are not.

Oh, I'd even argue against "amateur" and "pro" as being opposites. They are, after all, only distinguished by whether one receives payment for one's services (and "professional" usually means, to me, "makes a living wage"). Face it...we're "amateur" convention-runners, but that doesn't really make us the "opposite" of those who do it for money. (Just better able to complain that we're overworked and underpaid.)

Most of the professional writers (or ballplayers) we enjoy spending any time with at all are fans...they love the genre (or the game). Those with disdain for the genre are called Margaret Atwood, actually.