This was just something I brainstormed up throughout the last few days.

Sometime after the events of "To Where and Back Again," Celestia was arguing with the parliament not just over the new alliance with the reformed Changelings(Lead by King Thorax), but also Starlight Glimmer. The members of the Parliament consider the changelings too dangerous to be considered allies, and they consider Starlight Glimmer to be too much of a risk. They demand that Celestia hand over Starlight Glimmer for imprisonment into Tatarus, or she and Luna will lose their status as rulers of Equestria.

This leaves Celestia and Luna in a pickle: If they don't arrest Starlight and ruin Twilight Sparkle's chance of a successful apprentice, they will be Princesses of Equestria no more.

But things go from bad to worse when Chrysalis kidnaps Starlight Glimmer by posing as Twilight Sparkle. She then forces Starlight to resurrect King Sombra and break Lord Tirek from Tatarus. Learning of this, Celestia, the Mane 6 and Discord go to confront the terrible trio to save Starlight Glimmer. Celestia was able to save Starlight with the help of her companies.

Chrysalis, still hungry for revenge, decided to fire a dart of poison at Starlight. Celestia intended on taking the shot, only for Twilight to take the shot and the poison. This allows the Terrible Trio of Chrysalis, Sombra and Tirek to escape while Twilight struggles with the poison. Cadence arrives in time to see Twilight in a horrible condition, with a means to cure it: To be sealed away in Crystal so that the poison can be drained from her body.

Unfortunately, Celestia and Luna are called back to the Parliament after they learned that Starlight helped Chrysalis break Tirek out and resurrect Sombra. Despite Celestia and Luna knowing of what really happened, the Parliament demands Starlight's immediate arrest and imprisonment, no questions asked. Furious, Celestia and Luna surrender their crowns in protest and walk off. This leaves the Parliament enraged as they call out both former princesses of Treason, to no such luck.

As a result, Twilight Sparkle is now sealed in a purity crystal within the basement of her castle to save her of the poison, Celestia and Luna now make their home in Twilight's Castle as exiles with Celestia taking up the task of mentoring Starlight Glimmer through these hard times. Furthermore, Starlight Glimmer, Celestia and Luna now have a price on their head. Starlight for aiding Chrysalis unexpectely, and Celestia and Luna for treason.

The next episode after the two-parter deals with Starlight trying to adjust to life without Twilight while Celestia helps her new student out all while secretly transferring the School for Gifted Unicorns to Twilight's Castle. Word has it that the Crystal Empire has severed its ties with Canterlot, leaving Cadence as the only Princess left in Equestria. It ends with the Royal Guard, who were suppose to arrest Starlight, Celestia and Luna, laying their weapons down to side with the princesses-in-exile. The Wonderbolts also join the Royal Guard as well, much to Rainbow Dash's delight. This does not bode well with the Parliament, who now choose their new leader of Equestria to bring down Starlight, Celestia and Luna: Prince Blueblood.