The thing here is obviously Discord. I'm not even trying to hide it. However, Discord isn't going to be the conclusion of the story, but a catalyst for a question Celestia, and to a lesser extent Luna, have been dancing around all this time.

What is this world? Where did they come from?

Discord is quite obviously linked to the apparition and the dark magic in Celestia, and his appearance is going to shed light on what the dark magic is, how it ties in to canon, and how it ties in with the creation of the world and everything. What happens with Discord we already know from the history of the show, but he's not the climax, he's only the rising action to the final arc:
The Origin.