Taken from a previous post to some pony but I did not send it as it was out of context, we were discussing the possibility of Solar Flare (Celestia's version of Nightmare Moon) being the finale face off:
I have not been to comic con but I have seen the animatic too. A time travel device is not far fetched in equestria and I agree with you that Starlight Glimmer (with all of her stalking) have learned of Twilight's past and went back to stop the one key event that caused her to be the princess of friendship and meeting her friends.
Without the sonic rainboom, presumably Rainbow lost the race somehow and gave up her passion for racing and maybe, never got her mark or alternatively, the race to defend Fluttershy's honor did not happen and Dash never got to realize her passion.
Going with the latter, Fluttershy may not have landed on the ground and may have moved to ponyville to live quietly on her own but never discovering her connection with animals.
Without the rainboom, Pinkie would not have became a party pony and would have still lived on the rock farm with her family, maybe becoming more like Maud pie (personally, I love this idea as it would be fun to see the Pinkie and Maud act the same).
Applejack felt homesick regardless but never understood the feeling so she stayed with her aunt and uncle in Manehattan. Given from what we know about Manehattan, it is safe to assume that applejack became more of a city pony than country and her mannerism will be very different, as well as her ethics.
As for Rarity, she was literally dragged to the rock filled with gems but it never broke and so Rarity must have been left without her muse and her designer style of using gemstones. She might give up on her fashion and become like her parents.
As for Twilight, she gave up at the entrance exam and never caught the eye of Princess Celestia. So who would have taken that honor of being Celestia's prized student? I am not sure, maybe Sunset? or Starlight?
The nightmare Celestia is an interesting idea but I am not sure if we will see it at the end of season 5 as the card said "The day will last forever". It is the same as nightmare moon's ideology. Since Starlight is changing the Rainboom event, I am not sure if the ripple of time and space is enough to cause this to happen but it is a possibility.
For the finale, going off what would have happened if the rainboom never occured. I am not sure of Starlight's motive. Messing up the past of Twilight and the elements of harmony. Nightmare Moon is the first villain that I think of that would have ruled equestria and silenced Celestia without the elements to stop her. Then there is Discord, Sombra... so I guess that the writers have a balance factor so that they do not have to write such a complex tale. Maybe another set of the elements of harmony that differs from the mane 6?
Taking the idea from earlier, the unicorn that became Celestia student, became the new element of harmony's magic with a new set of 5 (as our current 5 is sort of unable to come to terms with their lives). They were the ones who saved equestria in the alternate world. Twilight... I am not sure how she would be as she is the main character, her memories will be intact.
As for Starlight's motive, she wanted to give ponies true friendship. I personally believe that she believes fully that equality represents friendship. This idea is more of a feeling that one would get after spending a long time being average or having seen how skills had divided others apart.
I am not sure how does the new mane 6 would fare but I guess that they did not go through the more impacting stories.
So Starlight would still have her village and slowly, she is gathering ponies into her cult. That might be her motive, to get back what she believed was real friendship.
It might be possible that Twilight and the rest are blank flanks or the mane 5 are and how they are not happy despite being equal. And also Spike would not be born in this timeline or he would have new mare to follow. Or Twilight and the mane 6 got their cutie marks in some way but they misinterpreted their marks that it made them feel what Starlight felt.
The animatic tells something about cutie mark magic and how Starlight was just sitting in the friendship castle. I still remember her line when her village was ruined: "Let's see how they like spending the rest of their days without their precious marks." This is her revenge probably.
On the topic of cutie mark magic, doesn't anyone remember Magical Mystery Cure? The spell that Twilight fixed that was originally Starswirl's unfinished masterpiece? What did the spell say again?
From all of us together, Together we are friends With the marks of our destinies made one There is magic without end
There is a high chance of this being something that might fix the mess that Starlight made. In season 5, we are starting to see more of background ponies being used to help the mane 6 and seeing more of foils for each of them. Also note the 3rd line: with the marks of our destinies made one, doesn't that seem like something like equality?
What do you think?