After season 6, I always had a feeling that Starlight Glimmer is the next Sunset Shimmer in Equestria while the original Sunset Shimmer is with the Equestria Girls franchise. There are signs of many similarities between the two characters despite being in different franchises and different life styles.
They both began their lives as ponies living in Equestria. Both of them were unicorns with great magic skills. Those two share a dark past. Sunset Shimmer was once Princess Celestia's star pupil before Twilight Sparkle, but when she didn't get what she wanted like being a princess, she turned evil and abandoned her studies. After that, she ran away from Equestria through the magic mirror leading into the human world and attended Canterlot High as a teenage girl. That was the last time they saw Sunset in Equestria until in the movie Equestria Girls, she stole Twilight's magic crown so she would claim what should've been hers. While in Equestria, Starlight was a fine little filly with great magical potential and has a good friend name Sunburst. One day, Sunburst has earned his cutie mark and left for Celestia's school leaving Starlight behind. Throughout the years, Starlight despite increasing her magic skills, became bitter and cold with a grudge on cutie marks. She ended up founding a village and stripped every pony's cutie mark to set up her equality scheme and spread it throughout Equestria.
Those two were defeated by Twilight Sparkle on different occasions, but later they show signs of remorse and have a second chance. Twilight along with the human counterparts of her pony friends defeated Sunset when she was in her demon form by using the power of her crown with the elements of her friends. When the smoke cleared, Sunset Shimmer started crying for all the things she has done, and accepted Twilight's friendship. She is then accepted y the humane 5 to learn more about the magic of friendship. Starlight on the other hand, did not have a chance of heart yet when she first met Twilight. At Starlight's village, Twilight and her friends foiled her plans and lost trust in her followers. She later escaped and vowing for revenge. She returns in the season 5 finale where she travels through time to prevent Twilight and her friends from having a connection by stopping the first Sonic Rainboom. But just as Starlight was about to succeed, Twilight shows her what happens if the Rainboom never happened, which will only lead to many disastrous events. After a few words from Twilight, Starlight finally starts to accept friendship and became Twilight's own student.
After being reformed, they have many struggles on friendship and being very fragile after what evil deeds they have done. They later started to step up to save the ones they cared and gained a boost of confidence. Sunset Shimmer was ostracized by other students at CHS, but when the sirens hypnotized the entire school, she over came her doubts and helped the Rainbooms defeat them and free the school. Later during the Friendship Games, Sci Twi accidentally steals and released the magic that turned her into Midnight Sparkle. Sunset stopped the rampage and save Sci Twi from her evil side. In Equestria, Starlight learned that her friends are captured by Queen Chrysalis and the changelings, and it is up to her with her own team to rescue them and save Equestria. Throughout season 6 however, she has been very uncomfortable and timid because of her actions, especially that she is struggling on being a leader to help her friends. She is now beginning to shine when she rescued the Mane 6 and the royal family along with helping Thorax show the changelings about sharing love which results in a major transformation that turns these creatures into colorful love-filled creatures and force Chrysalis out of her own kingdom.
You see, these two characters despite having different story grounds, has a lot of similarities. Both Sunset Shimmer and Starlight Glimmer were main villains before, and after being reformed, they both had their chance to shine and became heroes or protagonists. They share the same goal once they are reformed, and that's making their own worlds a better place with the magic of friendship.
So, what do y'all think?