I like this proposal a great deal, with one modification: In this model, "Fandoms by Source Community" might in fact be best labeled "Fandoms by Community",

Rationale: if one is going to be non-hierarchical, one should be non-hierarchical in all directions at once, and fan communities come in many sizes and shapes. Harry Potter fandom, filk fandom, and the SCA are large fan communities; at the same time, there are many small and oddly shaped fan communities centered around (for example) Wiscon, the Free Amazons of Darkover, and Yuletide.