I somehow misclicked save after only typing one sentence last time, so if you're wondering why you got two notifications, that's why. >_>
Overall, I liked this episode. It was nice to finally see some canon elaboration on the crucial backstory elements (NMM vs Celly, the sisters vs Discord). However, it's also co-served as a laser-guided spine-breaker for my fic! So, from this point on, my story is technically alternate universe; I may or may not choose to add the AU tag for this reason.
What exactly did the episode break?
1. NMM vs Celestia
Anyone with even a passing familiarity with my fic knows it's been broken from chapter 1, in which I showed the circumstances leading to the battle (and capped it off in chapter 2). Nightmare Moon in my story was born from dark magic that Luna gleaned from reading a forbidden text; later on the origin of dark magic itself, the god "Whisper", will be elaborated upon.
2. The sisters vs Discord
It's a long time coming, but my story will eventually show this fight via one of those memory crystals (like the one Celestia used to tell the story of King Sombra). In my story, the sisters were fillies at the time, and the battle is a lot more emotional because a) they've just lost their mother and b) they come to realize just what the elements mean, and actually mold them into their current form by identifying the six virtues they share.
3. The origin of the Elements
I'm sure season 4 hasn't told us the last of this; that six-socket box at the end is very obviously going to be an ongoing plot element that will probably break my fic even more when it's resolved (but hopefully I can have it finished by then). In my story the Elements of Harmony are the remnants of the god "Harmony", and were gifted to Celestia and Luna (as fillies) by a guardian alicorn god - Discord's lawful counterpart, in fact - whose identity I won't spoil here. My closer fans (Luminary, Mondai Shunketsu, Primrose Ishal and CartsBeforeHorses) already know about this anyway! These details will also be elaborated upon later via one of those memory crystals.
4. The mane six giving up the Elements
When my story jumps to the present day, the mane six will still possess them, because they will need their power to fight the aforementioned "Whisper." I may honor the canon development by having them lose the Elements at the end of my story, though. (For the record, the Tree of Harmony will not be included; it can't mesh with my story's internal continuity.)
5. The ability for Twilight to contact Discord via magic, and for the Elements to summon him to their location
This is a minor point that nonetheless caught me off guard. I'm not sure how I'm going to work with this; my original idea was for the mane six to have to track Discord down (leading them to Saddle Arabia). The purpose of this was to make for an amusing joke in which Discord pretends to be a genie, but I can always abandon this.
Does this mean I dislike the S4 opener? Not at all.
But I felt I might as well declare this now so that I can officially take my story off into AU territory with a clear conscience.
I hope you enjoy the episode!