When Twilight's portals release Tirek into the human world and the humans get turned into ponies, they have to unite with their ponyselves to defeat him and save both world's.
In an alternate timeline, somepony else got to the Mirror Portal before Sunset and stole both it and Celestia away. Acting Princess Cadance is willing to give Sunset a second chance, and hopes to have Sunset's help with a matter of national security.
Lemon Zest is pretty lucky for a pony who just got fried to death. She miraculously comes back as a ghost and stumbles on a haunted school where she can learn the basics of the afterlife. The fee of tuition? Recruiting more ghosts to school.