Sorry for butting in after reading only about half the conversation, so I'm probably missing some context, but I feel like fandom has given depth to a lot of characters (potentially a great number of female characters actually) that the show hasn't actually explored and I feel like my personal perception of their canon and their relevance to the mythology of the show has been drastically altered by their fanon development.
My favourite example is Missouri Mosely - her being so present in a lot of (I admit early season-based) fanworks and with a pretty consistent fanon characterization too has always made her into a very pivotal character in my mind, to the point that I was actually shocked when I went back to research something or other and discovered she really only ever appeared in one SINGULAR episode.
It's one of the things that I actually credit the show a lot with that with all their problems and pitfalls in representation on different sides of the ailes, they regular manage to create characters that might have little screentime but are at the same time so rich and full of potential, that they open up pockets of the shows universe for the fandom to latch onto and fill with amazing creativitiy. And in these instances I care actually rather little whether the source material delivers a more well rounded approach to these characters - and there are plenty of shows out there that do and I love those too - because I feel that in our interactions with it, we use these imperfections to make something great and worthwhile that a more sophisticated or perfect plot or characterization wouldn't open itself up to. That doesn't excuse bad writing or counting on fandom to fix it (and there's been enough of that over the SPN years too, which has annoyed me) of course, it's just a different perspective on the roads that don't get followed for one reason or another. Anyway, stopping my tired rambling now. *g*