Barbie gets PONIFIED! This is a cross between Barbie: the Princess and the Popstar / Barbie: The Princess and the Pauper. This is in the Equestria Girls universe and includes Alicorn Twilight.
Barbie stars as Rainbow Shimmer, (yes she's back from "The Secret Alicorn") the sports manager for Canterlot High. This means she organizes all the games and plays on all the teams. But she wants to have a life like Rainbow Dash's or Twilight's. Normality.
Rainbow Dash is a little jealous of Rainbow Shimmer's. She gets on all the teams because of her mother, Principal Celestia. She likes being the Captain of everything, but she wishes she could be like Rainbow Shimmer.
But the girls dream might come true... And switch out for one day.