If I may, I'd like to copy over my own idea for a "From Twilight to Sunset" scenario from elsewhere.
"A what-if story, where the branch-off from the original timeline(hereby called the Alpha Line) is where Celestia lets Sunset see what the crystal mirror was trying to show her, and it turns out to be a vision of what Sunset experiences near the end of the first Equestria Girls movie. The vision, showing what she'll become if she continues down her dark path, unnerves her and she refuses to have anything to do with the mirror. This prevents her from becoming disowned by Celestia and traveling to the human world, and stays on as her personal student. Eventually Celestia teaches Sunset about certain events of her own past, such as what happened with Luna(albeit with a bit of a fictional twist to hide the truth of Nightmare Moon's identity,) which helps open Sunset's eyes a bit in regards to her self-centered solitude, however she still goes on with her high-horse arrogance, albeit not so much cruel to others as before. Meanwhile, Twilight enters the school as a prodigy per Alpha Line, and Celestia decides to take her under her wing in order to give Sunset a possible friend. Eventually the two foster a sort of friendly rivalry, but mostly keeping to themselves. Thanks to this though, Twilight opens up a bit to other unicorns at the school, while Sunset stays with her magical studies since she's still quite self-absorbed. One day, she remembers the story Celestia told her about Luna and Nightmare Moon, and checks a book about prophecies(the same book Twilight read in Alpha Line.) And of course, upon trying to contact Celestia, she's told to stop studying magic so much and to go out and make more friends. Cue Episode 1: Friendship is Magic, as Sunset is sent to Ponyville instead of Twilight, who remains in Canterlot doing... magical things."