I assume Starlight Glimmer will be finally redeemed after her villainous effort to rewrite and alter Equestria's history in the final two-parter for Season 5, the Cutie Re-Mark.

I expect Fluttershy's brother to make HIS MLP debut in the two-parter at the beginning of Season 6. As Fluttershy's older brother, he originally was a mentor to both his younger sister and also to Rainbow Dash when they were still living in Cloudsdale.

By the time of the opening episode of Season 6, however, Flutterbro had turned to evil, as he willingly allowed himself to be taken over by a malicious weather spirit, granting Flutterbro the ability to generate violent weather including lightning, thunderstorms, tornadoes, and hurricane-level winds.

Flutterbro invades the Crystal Empire at the same time Princess Cadance is about to give birth to her baby pony, and her husband Shining Armor is too busy protecting her wife. The Crystal Heart cannot be used because Cadance is too busy in childbirth.

Twilight Sparkle, Fluttershy, Rainbow Dash, Rarity, Applejack, and Pinkie Pie use the Elements of Harmony to defeat Flutterbro, who is no longer possessed by the weather spirit and loses his ability to generate violent weather.

Flutterbro is sentenced by the Alicorns who rule Equestria (Princess Celestia, Princess Luna, Cadance, and Twilight) to community servitude under the supervision of the Mane Six, and even stripped of his ability to fly like other Pegasi. He is to be confined to Ponyville.

Over the course of Season 6, learning about the Magic of a Friendship, Flutterbro gradually turns to good, and eventually develops a close bond to Starlight Glimmer. They share a common heritage by which both ponies originally turned to evil, and now seek redemption.

By the time of the Season 6 two-part finale, Flutterbro has regained the ability and the right to fly with his own wings like most Pegasus ponies, and Flutterbro eventually marries Starlight Glimmer.

Season 6 episodes could feature flashbacks involving Fluttershy, Flutterbro, and Rainbow Dash. Perhaps a failure of Flutterbro to gain admittance to the The Wonderbolts started the path by which Fluttershy's male sibling eventually turned evil.

Other Season 6 episodes could feature Starlight Glimmer and Flutterbro interacting with Discord, the Cutie Mark Crusaders, Cheerilee and the young ponies in the Ponyville Schoolhouse, etc.