Okay, so I'm not sure this is gonna happen, but this is probably gonna happen.
Starlight's gonna go back in the past and stop the rainboom (the one due to which all the mane 6 got their cutie mark). The mane ponies won't get their cutie mark on time, or something else can happen, whatever. If Twilight doesn't get her cutie mark on that test moment, she wouldn't be Celestia's student. Then Twilight would never be sent to Ponyville, and never become friends with the Mane 6 (or maybe anyone else). If that doesn't happen, the elements of Harmony won't be discovered, or maybe just the last one. And Nightmare Moon won't be defeated and there will be eternal night. Later on, Discord, Queen Chrysalis, King Sombra, those black things which came from the Tree of Harmony and Lord Tirek won't be defeated. And imagine Equestria now, with all those people roaming free!
I am soooo excited for the finale!