Aletheia's

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
Yes, definitely! Actually this happens in any fandom, I think, and that's also one of main reasons I love the fandom culture in general.

Missouri seems almost entirely forgotten even in the fandom now, indeed. (Especially from my own perspective, a late-coming fan.) Well, admittedly it was inevitable with so many characters coming later and covering everything that was before in new layers of canon, so to speak. Then again, I'm still baffled at, say, Gabriel's career in the fandom, hardly proportional to what he got in the canon. That's why I'm a little miffed at the fandom for Bela Talbot's fate - her canonic coverage was comparable, same as her personality's power, and yet she's not even in tenth part as popular. For me, she's a fine example proving that fandom - every fandom, not only SPN - should look also at itself, before it accuses the creators of misogyny and other prejudices. Then again, love for a character isn't what one can just switch on, so it's not like I really demand it from anyone. I just think some people should be less narrow minded in their accusations and grudges.

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.
Yes, and what's more, they actually appreciate this dialogue with the fandom, and recognize its contribution, rather than condemn it (as many other creators in the past and now). I'd say currently it's maybe the most important and characteristic thing in the whole SPN phenomenon. As for bad writing - on the one hand I'm one of those fans loving to pick at plot holes and grumbling about quality and such, but on the other hand, I think many fans tends to forget the show is being made by a (big!) team, not just one person. Inconsistencies - in plot, in characters, in worldbuilding, in messages - are to be expected. Of course, I totally agree with you this doesn't excuse fails, but at the same time I think those fails don't really deserve such extremal levels of outrage I often see... But, well, that's also one of things that define fandom - extreme feelings. :)

Thanks for coming and taking your time to write a long comment! I appreciate it! :)