Several episode synopses have been revealed.
S9E01&02 "The Beginning of the End" Apr. 6, 2019 - Episode Discussion:
Spoiler
In the back-to-back season premiere episodes, titled "The Beginning of the End," Princess Celestia and Princess Luna decide to retire and hand off the ruling of Equestria to Twilight Sparkle and her friends, which sends Twilight into a tailspin of self-doubt. Meanwhile, an ancient villain bands together a legion of nefarious characters to conquer all of Equestria, challenging Twilight Sparkle and the rest of the Mane 6 to step in and save the day.
The Beginning of the End, Part I When Celestia and Luna decide to retire and hand off the ruling of Equestria to Twilight and her friends, it send Twilight into a tailspin of self-doubt. The ancient villain Grogar plots to conquer all of Equestrian, but King Sombra has ideas of his own.
The Beginning of the End, Part II As Twilight and the rest of the Mane six struggle to come to terms with destruction of the Tree and Elements of Harmony, King Sombra sets his sights on conquering all of Equestria.
(sources: not specified, info. from EqD)
S9E03 "Uprooted" Episode Discussion:
Spoiler
The Young Six respond to a magical summons from the Tree of Harmony, only to learn it has been destroyed.
(source: https://whensiton.com/episode/uprooted/EP013304160240)
S9E04 "Twilight's Seven" (200th Episode) Episode Discussion:
Spoiler
Twilight and Shining Armor Pit their wits against each other to settle a long-standing sibling rivalry, but they soon discover they are not the only competitors.
(source: whenitson.com)
S9E05 "The Point of No Return" Episode Discussion:
Spoiler
Twilight realizes she never returned a book to the Canterlot Library and may have caused her favorite librarian to lose her job.
(source: whenitson.com)
[ Moderator Note: In order to reduce clutter, all FIM speculation threads will be condensed into this mega thread. Please refrain from making official episode threads until the episodes actually begin airing in April. Thanks. - Bekuno ]