Here is my Alternate Universe idea, but before I go to the Mane 6 and the Princesses, I'll start with Starlight Glimmer. Why? Read on to find out!

When Starlight was only a filly, her friend Sunburst left her because of his cutie mark (that sticks to the show), to attend Princess Celestia's School for Gifted Unicorns. Starlight was still a 'blank flank' back then, and was often teased and bullied by others in her school. As she grew older, she created a village which is equal, to create no future pain for herself. (Sticks to the show there)

As Starlight spent more time in her village, she realized her magic potential was really high, so she tried experimenting with high-level magic. She pursued knowledge and power at all costs, finally turning to the Alicorn Amulet for a boost of power. The Amulet corrupted her, amplifying her bitterness and corrupting her. As she started to experiment with dark magic, Starlight turned to Queen Chrysalis, King Sombra and Nightmare Moon (Actually, what corrupted Luna into transforming into Nightmare Moon) for guidance. Chrysalis, Sombra and Nightmare Moon saw the potential in the mare, and taught her various kinds of dark arts, gradually transforming Starlight into a tyrant.

When her abilities are mature, Starlight was transformed into an Alicorn by her mentors, and began her quest to conquer Equestria. She stormed to Canterlot, where she captured the Princesses (Celestia, Luna ad Cadence. Twilight isn't an Alicorn in this AU here yet), taking their magic and banishing them away to a place faraway. Hence, Starlight claimed the throne to Equestria, found the Elements of Harmony, and shattered them all into various realms in Equestria. Remembering her village, Starlight established a no-cutie mark republic, replacing the marks with equal signs. Those who submitted are allowed to roam free, but in great fear, and those who don't are imprisoned until they do.

Okay, the Tantabus/King Sombra/Queen Chrysalis/Alicorn Amulet parts look weird…