Starlight is like Sunset when they both started their life as simple unicorns with magical abilities. Before the series starts, they both have gone to the dark side. Starlight lost her friend, Sunburst when he got his cutie mark, that what makes Starlight bitter about other ponies' cutie mark believing they are way special than her. While Sunset Shimmer on the other hand, became cruel when she did not get what she want as quickly as possible, and she ran away to Canterlot High in the human world where she became a bully making most students afraid. Everything soon changed when Princess Twilight Sparkle came into the picture. She and her friends from both Equestria and the human world were able to stop their evil plots.
They way on how these two antagonists were defeated during Twilight's first encounter were somehow different from each other. Sunset Shimmer in her demon form was defeated by the Elements of Harmony. It helps purged Sunset's bitter and make her open her eyes while crying and show regret for what she did. Back in Equestria, while Twilight first Starlight at the equal village, she was stopped, but she immediately didn't show remorse for her actions, denied a friendship lesson, and in the end, she escapes to one day plan her revenge. After being reformed in Equestria Girls, Sunset starts to play a bigger role in her part as a hero in Rainbow Rocks and Friendship Games. She helped the Rainbooms defeat the sirens after stepping in to to keep the friendship together and stop Midnight Sparkle's rampage and turn her back to a friendly Sci Twi. Princess Twilight meets Starlight Glimmer again, and this time Starlight is looking for revenge. Starlight wants to prevent the first Rainboom from ever happening so the Mane 6 would never earned their cutie marks. Unfortunately, things didn't turned out well for anypony, when Twilight went to multiple timelines, Equestria has become a place of horror with different villains reigning supreme. Once Twilight tries to convince Starlight the her actions in the past can doom Equestria, Starlight refused to listen at first. Later, when Starlight was going to make a doomed timeline permanent, Twilight reaches her out to her with a long friendship talk and how important it is for all of Equestria, making her slowly opening her heart and accepted friendship.
Following the aftermath of different conflicts, both Sunset and Starlight are accepted into the group. In EG,human Twilight transferred from Crystal Prep to CHS to be with Sunset and the Mane 5 humans to form the Mane 7. While in Equestria, Twilight and her friends decided to keep Starlight her in Ponyville after concerning she is too powerful to just let her go. This could be a possible Mane 7 of Equestria with Starlight as a student to learn more about friendship.
Overall, this made me think that Starlight Glimmer is the new Sunset Shimmer in Equestria while the real Sunset Shimmer is part of the Equestria Girls franchise. I hope one day we see more of Starlight Glimmer having adventures with the Mane 6 and her new life in Ponyville in season 6.
Please comment on what you think.