Memories of a Lost Sun: When the Memory Stone is destroyed, everyone’s memories are restored. However, something backfires and instead of regaining her lost memories, Sunset Shimmer loses all her memories including her own name. After her friends fail to help her remember her lost past, Princess Twilight Sparkle resorts to finding Sunset’s family. When they fail to help, Sunset’s distrustful parents, blaming humans for all her misfortunes, believe the only solution for Sunset to have a good life is to bring her back to Equestria forever. Everyone at CHS becomes very sad at hearing this, especially when they hear that Sunset’s parents have arranged for her to be married when she returns. Wallflower Blush, the culprit of the entire thing, feels especially guilty for the damage she caused. It gets even worse when she finds out what kind of person Sunset’s fiancé is. Now Wallflower must convince Sunset’s friends to help her undo what she’s done before they lose Sunset Shimmer forever.

Original character summaries:
Blaze Sunrise and Shimmering Dawn: Sunset Shimmer and Sunshine Summer’s parents. Two ponies of Canterlot’s noble class. They are classy but also very strict, distant, and serious. Their relationship with their eldest daughter isn’t the best. They had very high expectations of Sunset and put a lot of pressure on her to be the best when she was a filly. When she became Princess Celestia’s pupil, their expectations became greater to the point where they became even more distant and didn’t even communicate for years. The news of Twilight Sparkle becoming Celestia’s new pupil and Princess of Friendship greatly disappointed them. Despite their aloof nature, they really do care for their daughter and want what they think is best for her. The news of Sunset’s complete memory loss greatly upset them. They have trust issues and look down on those of lesser class and species. They do not believe Twilight is worthy of being a princess or even a friend to their daughter, and also blame her for most of their family’s troubles. Hearing what Sunset has been through caused them to develop a very low opinion of humans, especially the Humane Six and Wallflower. These trust issues are also their greatest weakness as they make them very easy to manipulate.
Sunshine Summer: Sunset Shimmer’s younger sister. She’s just a few years younger than the Crusaders themselves and is childlike and naïve in nature. Despite all the things her sister did in the past, she looked up to her sister and always thought of her in a positive light. The news of what happened to Sunset greatly devastated her. Like her parents, she becomes very resentful and hostile toward humans blaming them for all the misery that befell her family to the point of seeing them as stupid, selfish, and pure evil monsters. Despite that, all she really wants is for her beloved sister to remember who she is and come home.
Troy Horace: a young stallion of Canterlot’s noble class, a pony from Sunset Shimmer’s childhood, and now her fiancé. His family were friends of Sunset’s family so he is on good terms with her parents. He presents himself as a caring, reasonable, and noble person but is actually a cruel, despicable, and manipulative sociopath who wants to gain access to the family’s wealth and power. Neither his family nor Sunset’s family know of his true nature as he was able to convince Sunset’s parents to arrange her marriage to him. He even manipulates Sunset’s sister into hating her human friends. He lies about being an old childhood friend of Sunset and caring for her well-being when in reality he used to torment her when she was a filly, making him the biggest reason Sunset became the bully she used to be. When news of Sunset’s complete amnesia reached him, he was excited. With Sunset’s inability to remember the kind of person he is, he saw this as the perfect opportunity to put his plans into motion and will stop at nothing to achieve them by any means necessary. He is the main antagonist of this story.

Please leave thoughts in the comments.

Why aren't you writing this already?

That first one sounds interesting