Twilight, in the usual fashion, gets sent (read exiled, not to return until her mission is complete) to the humanoid world.
A scene in the trailer shows no Twilight on a group photo. Pinkie Pie does not acknowledge that that is actually her name. What if Sunset Shimmer was this alternate universe’s Twilight Sparkle? They both share a similar definition between their names. Sunset, Twilight; Shimmer, Sparkle.
Twilight falls head over heels, literally, to an unknown male humanoid. His cutie mark is a Lightning Bolt on a Shield. So he ain’t Shining Armor or Soarin, but could he be Sunset Shimmer’s brother?
Sunset Shimmer explodes in a bright light much like how a changeling did during the Canterlot Royal Wedding. What if Sunset Shimmer wasn’t Sunset Shimmer at all, but a changeling? What about Queen Chrysalis?
Queen Chrysalis has the ability to lock up her victims in pods of goo as seen with Princess Celestia and in comic canon with the Cutie Mark Crusaders’ disappearance. What if the real Sunset Shimmer was abducted, changing the course of time to create a world without a Twilight equivalent?
Hasbro wouldn’t market a toy based on a villain, and we’ve seen the rumoured Sunset Shimmer toy, but there hasn’t been a King Sombra or Queen Chrysalis toy. Even Princess Cadance, who turned out to be Queen Chrysalis all along was also a toy as her true self.
To summarise: Twilight has to stop the evil force of Queen Chrysalis to bring together the Main Six of the humanoid world and stop her once and for all. The End.