To those saying Rarity would be an antagonist if this had been any other show, I’ve got six words to prove you wrong:
 
Daphne Blake
 
Morticia Addams
 
Princess Peach
 
 
Those three characters represent archetypes that take up a third of her personality.
 
Daphne - the fashion savvy optimist, kind hearted and considerate.
 
Morticia - the artistic one - passionate and talented
 
Peach - the sweet hearted one - generous to fault, energetic, adventurous, and polite.
 
All three of these characters are protagonists in their respective series, except Peach who’s actually the tritagonist with Mario as the protagonist and Luigi as the deuteragonist.
 
If the show was more like Scooby-Doo she and Twilight would be deuteragonists, while Fluttershy and Pinkie are the main protagonists with Spike rounding out the main cast as the tritagonist and Rainbow thrown in later to boost ratings.
 
If it was The Addams Family, She and Spike would be the main characters.
 
The character more likely to be an antagonist is Trixie and that’s exactly what she is in this one.
 
Rarity on the other hand would be a main protagonist at most likable and a barely relevant background character at worst.