I'm on Team Fanon in the Harry/Draco Worldcup tournament, and we had a discussion regarding what Fanon is--thought I'd share.
I think the beauty of "fanon" is how broad an interpretation it can be. For me, fanon can be considered anything that didn't happen and isn't likely to happen. And now that canon is closed, how wide a range that is.
-is an AU and/or departs from canon at some point (e.g., EWE)? Fanon.
-is an AR (alternate reality)? Fanon.
-is a "behind the scene" from canon, but who can say if it really happened like that? Fanon.
-contrasts something we're meant to assume about the essential nature of the characters (e.g., their sexuality)? Fanon.
-takes something canonical and weaves an explanation that debunks the plain-and-simple explanation for it (e.g., affection, death, etc.)? Fanon.
-narrates canonical events from another point of view, delving into the brain of a character whose motivations we can't truly be said to know? Fanon.
-speculates on the canon's future in a way that the original author probably did not plan for the future to go? Fanon.
Fanon is...what a fan creates.