Canon marches on · 3:28am Oct 30th, 2015
When I started writing Chitin, I wanted to write about ideas and concepts raised by the first two seasons, and I had zero interest in writing about season 4. I felt like I needed to acknowledge more recent developments, so I made it an AU splitting off at season 3.
As part of my messing around with ideas from the show, I intentionally chose character interpretations that were plausible, but were unpopular in the fandom. For instance, Rainbow Dash was the most popular character to write as trans, and Fluttershy was almost never written as trans but could theoretically be interpreted that way, so I made Fluttershy trans and Rainbow Dash cis. I knew my story was going to become more and more AU as the show continued, but I figured I'd have it finished before too long anyway.
I didn't realize just how swamped I would get at school, and I didn't think about how hard it would be to write the story after so long away from it. Canon has changed so much since I stopped, and at least two characters' entire role and purpose in the story is contradicted by what we learned about them in season 5. I wanted to include all the new information we got, but I didn't know how to integrate it into the plot I already had planned, and I found myself paralyzed.
It was actually Steven Universe that got me thinking again. Pearl's great with machinery, so fic after fic assumed she built robots or repaired computers before the war. Then canon suddenly revealed that she was a domestic servant whose primary duty was to stand around and look pretty. Ongoing fics continued to portray her as having worked with robots or computers, because that was the background they'd already set up.
I'm still deciding what to do with Chitin, and I think I'll do what I can to integrate new aspects of canon. But I won't worry too much if I can't make a season 3 idea work with season 5 information. It was always supposed to be a different spin on Ponyville, and I don't think folks will mind too much if it winds up more different than I intended.
I've always figured that there's no point in trying to keep up with future canon when you're writing a divergent fanfic. It becomes an AU, where your headcanon is the rule.
You worry too much. Just slap it with the "Alternate Universe" Tag and be done with it!
I think it's a good approach to take what you think would improve the story but not worry about cramming in something just because it's in the show, yes.
This is always a big risk in fanfiction, and readers are understanding of it. I think the writers care far more about the canonicity of their stories than the readers do, so I really wouldn't worry about it. Just go ahead with what you had planned, or include whatever changes you want to include that won't make a mess of things for you.
That's the beauty of the AU tag. Internally consistent ideas can still function, even if they diverge from canon. I look forward to more of this setting.
Speaking personally, I don't consider it AU if it was consistent at the point you wrote it.
For example - twilight as an only foal, or as a unicorn, or living in the Golden Oaks.
History is not AU.
Any other perspective would mean that essentially everything that interacts with anything ever on screen would be AU, which is a bit ridiculous.
3507296 This right here.
Anytime you write fanfic in an ongoing universe, you have to expect it will get Jossed sooner or later. As long as it was consistent with the canon as we knew it when you began I don't consider that to be an AU.
It's like I always say, "When in doubt, chuck canon out the window and watch it go splat."
I'm just going to link this here, because it's great and everything I want to say at times like this.