This Theory is about Sunset Glimmer on the season 5 opener being Diamond Tiara's mom. Starlight left Filthy Rich when Diamond Tiara was a baby foal, and became the leader of Our Town. Diamond Tiara could've inherited her antagonistic traits from her.
Both have similar coat, and mane color
Both are against cutie marks(Starlight Glimmer with believing that true friendship comes from being the same, and Diamond Tiara being mean to the CMC for having no cutie marks)
Both are villains