i'd want to note that the push for social justice in fannish work is a relatively recent, couple of years old thing (if we don't count slash itself as such), and it's mostly moves from meta to works, not the other way around. though, of course, certain percent of fannish works is and was always pointed to challenging canon, questioning it (did you see women's work, by the way? it's a pretty amazing meta-vid).

i'd say fanfic is not as much about being tied to source text - you can read stuff in fandoms you know no further than wiki page about (some write like this too, though it's considered to be in a bad taste) as about tied to overall fannish context, some sort of meta-language of fanfiction, set of conditions and parameters and cliches everybody agrees to. there's something... immediately, stylistically spotable about fanfic, whatever genre it's in, though it's hard to quantify.