I know, the Season 8 Finale won't be till sometime in October. But I would like to speculate on how I think it may go down, considering that
a: Cozy Glow is slated to be the main antagonist
b: Lord Tirek will return(Confirmed by his actor Mark Acheson), most likely to pull off the HijackedbyGanon trope
c: And above all else:
A leak showcases a stone statue of Tirek giving Cozy Glow a piggyback ride, hinting that these two will reform
Now, since the beginning of the year, I've been speculating on how this will all play out, and this is my personal theory. Note: When the episode comes out, you may disregard this post, unless it somehow ends up being true.
When magic disappears, Twilight, Starlight, Spike and the MAne 5 leave the school of Friendship to pursue this incident, going out on adventure for the first time since the My Little Pony Movie(For Starlight, it's since Shadow Play)
Cozy Glow, who sees an opportunity, strikes and begins to ambush the school of friendship with some magic that she acquired, forcing the Student Six and the Cutie Mark Crusaders to intervene.
Meanwhile, the Mane 8 finally discover who's behind it: Lord Tirek is back. He proceeds to drain them of their magic, then seals them up in Tatarus. He contacts Cozy Glow and learns that everything is going according to plan. But as Tirek does glow, he seems to smile abit over Cozy Glow, like they had formed a bond of friendship.
As Cozy Glow and Tirek began rampaging across Equestria(And the School of Friendship), the Student 6 and the CMC serves as the final bastion against the duo while the Mane 8 fights their way out of Tatarus. After a long struggle, they escape and return to Equestria to confront Tirek and Cozy Glow. The Mane 6 were able to acquire their Rainbow Power and fight Tirek. But he has gotten smarter since his last encounter, all thanks to Cozy Glow.
But in the middle of his rampage, Tirek notices something wasn't right. Somehow, he has critically injured Cozy Glow, and left her near death. Unlike with his experience with Discord, something within Tirek finally snaps and he realizes what he did wrong. In what may be his first act of generosity, Tirek gives up all the magic he acquired to save Cozy Glow, much to the surprise of the Mane 8, and even Discord, who's still got a beef with the CEntaur. After Cozy Glow was brought back to life, Tirek surrenders himself, having finally felt shame for what he did.
But believing there may be a way to change Tirek, Twilight was able to consult the other princesses, and they agreed to give Tirek a chance to prove that he is willing to walk the path of redemption. But one Chancellor Neighsay comes by, objecting to this. He points out not only because of what he did, twice. But also because of recent events like the STorm King, or Queen Chrysalis.
Then it was revealed that he opened the doors to Tatarus, after Tirek mentioned that it was easier to escape. This revelation has all the princesses furious, and they strip him of his status of Chancellor of the E.E.A.
As the months go by, Twilight and her friends sing a song of progress as the Student 6, Cozy Glow and Tirek continue their studies in the magic of friendship. The climax of the song involves all of them graduating from the School of Friendship.[/spoiler]