The idea goes like this. And it would be a 2 hour special.
Twilight Sparkle has decided to put on a stage show in a brand new stagehouse theatre. She hires a number of familiar characters, including her friends, many of the familiar side characters, and even the villains. Like with the Muppet Show, there are a number of random skits that are based on certain characters. There is even going to be mash ups on random characters.
But as a part of the side plot, the Villains are scheming to overthrow Twilight Sparkle and take over the Stage House just to put on their own shows. The villains are
Princess Luna/Nightmare Moon
Discord(Ringleader)
Queen Chrysalis
King Sombra
Trixie
Sunset Shimmer
Lord Tirek
The Dazzlings(Adagio Dazzle, Aria Blaze and Sonata Dusk)
Starlight Glimmer
Gloriosa Daisy/Gaia Everfree
Stygian/The Pony of Shadows
Cozy Glow
Princess Celestia/Daybreaker
But they also have a mole within the Mane Six. And they have a regular contact with this mole off screen. No one in the audience knows who this mole is, though it is hinted to be Fluttershy.
Near the end of it all, the villains turn on the Mane Six and Spike, revealing their plan to conquer the stage house. Twilight and her friends are even more appalled that Celestia, Luna, Sunset, Starlight and Trixie would take sides with the other villains, with Fluttershy being upset at Discord staging it all.
But the biggest plot twist is: