What if Starlight goes back and steals the mark and power of Nightmare Moon just before the fight is supposed to take place in Ep2?  
She could then either, go into hiding with the powers, greet Celestia and try and take Twilights place by friendship, or try and take Celestias mark and power.
 
As there seems to be a replacement castle in the game Starlight will probably fill Twilights role closely enough to gain a tower.

@Sejen  
Starlight isn’t stupid so that would prevent the apocalypse from happening if the Mane 6 weren’t there to stop it.

What if the plan isn’t to steal Twilight’s cutiemark or destiny.  
Perhaps Starlight plans to go back in time and steal the mark and powers from Nightmare Moon?

And then, Darkness.
 
“Once upon a time, in the magical land of Equestria….”

@Sejen  
Spike: Is this Equestria or is it Tartarus?
 
Twilight: It’s Equestria alright but I can’t imagine Tartarus being much worse!

Lol this was pretty accurate.

@Sejen  

 
That or they put Starlight in a cell where Discord stabs her to death.

[Facehoof] Oh my! It’s all so clear to me now! It’s “Back to the Future Part II” with the feels and morals of “A Christmas Carol”!
 
Theory:  
As Starlights cutiemark/special talent is in cutiemark removal she’s probably going to go with a supercharged version of her removal spell. In this case it removes the mark and the “backstory/destiny of it, she wants it for herself!
 
Twilight is going to cast some kind of spell, (teleport perhaps?) this is going to put her “out of range” of the changes.
 
Starlight will go back and try to live Twilights life by faking friendship (with the real Twilight as a ghost?).
 
Starlight, works her way to the present and fails to make friends with the gang in Ponyville (she got the mark by cheating and doesn’t fully understand the rules of getting/being friends).
 
Stuck in the “bad end” (eternal night) Starlight now realises friendship has to be genuine and reverses the spell with Twilight.
 
Now back in the good present Twilight uses the magic mirror.
 
Of course you’ve probably figured all this out yourselves…

Starlight is trying to change the past!? THIS IS CRAZY!!!!!

If Starlight plans on Changing the Past, then here’s my predictions:
 
Rainbow Dash, she ends up almost crashing into young Derpy, cancelling out the Sonic Rainboom.  
Fluttershy, ends up so in love with the ground that she is oblivious to the creatures being wild, and either ends up wanting them killed, or is scarred for life, after being terrorized by a bushel of berserk bunnies.  
Rarity, she lets her school down, and ends up becoming a bitter disappointment, as well as becoming a crazy cat lady, as once prophesized by Pinkie Pie.  
Applejack, never leaves Manehattan, and changes her name to Apple Jewel, a city debutante who was raised by her Aunt and Uncle Orange, and leaves Big Macintosh to suffer the work load dumped onto him. Between helping Granny Smith, raising Apple Bloom, and doing the chores around the farm, Big Mac ends up overworked, overwhelmed, and overstressed!  
Pinkamina Dianne Pie, never leaves the rock farm, and becomes boring, depressed, and ends up making all of Equestria sad, due to not being a bundle of joy.  
And finally, Twilight Sparkle, ends up failing the Entrance Exam for Celestia’s school, doesn’t get her cutie mark, and ends up wasting Celestia’s time, and is sent to Magic Kindergarten.
 
And also, Celestia herself, ends up filled with feelings of regret amd disappointment, ends up banishing Nightmare Moon back into the Moon, AND sends the moon away, after being corrupted by the POWER of the Elements of Harmony, and she herself becomes Nightmare Star, an evil not unlike Master Eraqus, who was tainted, not by darkness, but by light!

I know what will happen…
 
In the grim darkness of this new future…
 

 
…there is only… WAR!
 
Thanks to Starlight’s altering of the timeline, Equestria stands on the brink of annihilation… The noble Solar Imperium is the only force standing to defend it, under the banner of the God Princess Celestia… The Imperium is surrounded by threats on all sides–from the ruined lands of Griffonstone, the savage and greedy griffons; from the volcanic wastes of the Ashlands, the powerful and bloodthirsty dragons; from the hives hidden deep in the Everfree Forest, the deceptive and manipulative changelings; from the returned Crystal Empire, the dark Umbrum hordes of King Sombra; from the western deserts of Appleloosa, the mighty buffalo tribes; from the eastern tauric kingdoms, the power-hungry minotaurs; from the pits of Tartarus, the wicked Tirek together with the newly freed Discord, and their armies of the Chaos Legion; from the moon high above, Nightmare Moon and her Lunar Alliance, seeking to drown Equestria in darkness for all time; and from northern Equestria, fanning the flames of war from the shadows, determined to claim the spoils from the winner, Starlight Glimmer and the full military might of the Equalist Union! Only one pony can save Equestria now; a blue unicorn taught by the God Princess herself! This Great and Powerful unicorn must lead her chapter of Celestia’s Ultramarines to absolute victory, or face the complete destruction of Equestria as she knows it! They fight for the God Princess! And they shall know no fear!!!

@Psyga315  
Well see, I didn’t know that.

@Background Pony #5572  
No. Just the dialogue is fake.

Fake tag? Is this fake?

@Dogman15  
Which would be amazing considering I borrowed and re-cobbled Starlight’s line from a TV show.

@thps48  
The video/animatic preview that was shown at Comic Con is not a leak, true.  
But if the text in this particular image turn out to be what is actually said later in that scene, then these words would be considered a leak, since they came from an unverified and unofficial source.

@Background Pony #A34C  
Some joke about Josefumi Kujo. lol

@OptimusPrimevil  
If Starlight does this in the season finale, then she would really go from stealing abilities to ‘universe reset’.  
…  
MLP FiM Season 6: Twiley Sparkle competes against Beatrix Lulamoon in the Running of the Leaves across Equestria and in collecting the Elements of Harmony. Meanwhile, Celestia is doing unsavory acts at a discount.

@Dogman15  
It’s. Not. A. Leak. XP

I will laugh if this turns out to be what is actually said, because that will mean we had another glorious leak on our hands.

she’s not going to cast a spell…she’s going to use a [STAND]

I personally will be disappointed if when she cages the past, if that really is what’s going to happen, that they don’t include everything that would’ve affected.
 
I.E. Nightmare Moon is still around, Spike didn’t hatch, and that’s not even counting the stuff that would’ve happened later.

@Toxicpinkiepie  
Perhaps there will be a “replacement service”, a different unicorn is “magic”? Or perhaps Glimmer planned on being Twilights replacement?

Tes, but doesn’t this mean every villain will get a redo? and if Twilight is out of the picture then all the baddies are gonna have a picnic.

Starlight is the true Friendship Nemesis. Someday she will extract the mind failures from Luna & Celestia and will use it to overthrow Equestria in her hooves. Prey Starswirl the Bearded that Sunset Shimmer will return to form the Mane7 in Equestia verse.