Rainbow Dude
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Hmm… Yeah, that does seems to be something…  
But it may not be…
ArrJaySketch
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The End wasn't The End - Found a new home after the great exodus of 2012

I’m not surprised, considering there’s a whole school for gifted unicorns. We’ll have to see how it all plays out.
 
Even if she never appears in the show, this alone could be a fantastic setup for a bunch of a fanfics/comics.
Background Pony #6F69
What if she’s Princess Celestia’s daughter?
Masem
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@Background Pony  
That nicely sets up the jealousy that would be a good story driver for the core FIM message. (plus about a bazillion fanfics that would go well beyond that - shades of the Star Wars prequels, for example).
 
I doubt we’re getting her as a Mane 7 but as a recurring character like Cadance and Shining.
Background Pony #8457
What if she was Celestia’s student until Twilight proved herself back in Cutie Mark Chronicles?
Krek
The End wasn't The End - Found a new home after the great exodus of 2012

@Background Pony  
Would it be possible or feasible for a possible “Mane 7”?
Background Pony #C3D9
@Background Pony
 
And then it turns out to be a trap and sunset shimmer is a fake and turns into queen chrysalis.
Background Pony #8F4C
At first I was afraid that she was going to replace Twilight. However after several confirmations from Tara and Meghan and some other merchandise showing Twilicorn between her friends, I’m pretty sure she won’t be replaced. Now I’m actually curious about what part she might play in season 4 or maybe she’s just a toy after all.
Hippob4

@Background Pony  
But (at least, so far) we haven’t seen any Twilight toy with the mask. Only SS and the Mane Five. Maybe this ‘Adventure’ two parter will be a Twilight-less one, for a change?
Background Pony #D062
I’m casting a vote for her being perfectly happy and well-adjusted with no villainous feeling towards Celestia and company whatsoever. She asks Celestia for help in some problem she’s facing, and Celestia sends Twilight’s friends – and possibly also Twilight herself – as that help. In the process, they wear masks.
bboifatz
The End wasn't The End - Found a new home after the great exodus of 2012

Celestia originally picked her to be her successor, but she told Celestia to shove it and quit the school.
DragonRanger
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I’m envisioning her as being something of a gifted but lazy student. She shares Twilight’s magical potential, but preferred thrill-seeking to studying and learning magic.
Background Pony #2BC1
Yeah, this pretty much clinches the idea that she’s going to be a character in season 4. The question now is “how.”
 
That ‘years ago’ thing makes her sound old, but Hasbro likes to keep their characters younger to be more marketable. My bet is that it’ll be some sort of Crystal Empire deal where she was sealed away and then comes back as if no time had passed.
Masem
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@Star Cavalcade  
Possibly though consider Celestia technically didn’t give Twilight a choice about becoming an alicorn.
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Hoof Boi
Nice… I can’t wait! :D
Star Cavalcade

@Masem  
Maybe she was and just didn’t want to be a princess and be tied down with ruling a kingdom?
Masem
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If we take that Celestia has been looking for a pony powerful in magic for years, and took the most promising as her students to mentor directly, Summer Shimmer could easily be one of those.
 
Given that this is the Hasbro version, I can’t see it being villainous (eg think Twilight’s version of a Lightning Dust counterpart). Perhaps after Celestia was assured Summer wasn’t the powerful magic welder she was looking for, taught her well enough and sent her off on her own in a harmonious manner.
 
I’m sure the fanfic community will make her the evil displaced student come back for revenge, but I can’t see the show going in that direction. Though I can see some antagonism between her and Twilight that would lead to decent stories (better than the outright rivalry of Twilight and Trixie).
Obsessive Rarity Fan
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This is going to be interesting…
GLord
The End wasn't The End - Found a new home after the great exodus of 2012

@cyanfunk
 
If only the conversation would go like that.
ClownDicks
The End wasn't The End - Found a new home after the great exodus of 2012

@HyperNerd  
You mean like Skyla wasn’t just some HasbrOC?
Merrimack

Alright, let’s plot this out:
 
  • Once upon a time, probably 1,000 years ago, Sunset was Celestia’s student  
  • Sunset did something evil/forgot to turn her homework in on time and was sealed away for a millenium  
  • Sunset swore total revenge on Celestia upon her release  
  • Celestia naturally forgot about all that (she’s old, cut her some slack) and is actually surprised when Sunset appears before her  
  • “Sunset? I thought you died a thousand years ago!”  
  • “You sealed me away, jackass!”  
  • “Hahaha…yeah, I tend to do that. So have you met my new student, Twilight? I made her a princess, you know!”
GLord
The End wasn't The End - Found a new home after the great exodus of 2012

SUCK IT! SUCK IT!!!
 
Okay, still not conclusive proof she’s gonna join the mane group, but still, SUCK IT!!
HyperNerd

Okay, from that description, I’m near sure that she isn’t just some HasbrOC who will never appear in the show. She will almost definitely appear in season 4. Perhaps she will be a villian jealous of Twilight’s ascension to the throne?