One thing I've noticed in fandom over the years is that certain ideas or fill-ins tend to turn viral - fanon results, and you have people picking up things that make leaps and connections not explicitly addressed in the main text and going "well, of course it's like that." I kind of love the "group mind" we create around fandoms, it's a huge part of what makes it fun for me.

(The best example of this for me isn't actually a fanon thing, but rather the first Harry Potter movie's soundtrack. The books don't literally have music, of course - there was nothing in any of that text to indicate or suggest a certain chord or piece of music in any explicit way, but as soon as I heard John William's score, my thought was "yes, this is *exactly* the music, of *course* it is, this is RIGHT." I will have that same reaction reading a fic or a comment or meta sometimes. I just get this sense in my head of "yep, that's exactly how that works.")