Since My Little Pony Equestria Girls: Rainbow Rocks isn't out yet, I figured everybody should post their guesses on what might happen in the movie in the comments.
This is my guess:
Judging by the third sneak peek clip, Rainbow Dash's ego has made her an oboxious leader of the Rainbooms. The others complaints may cause a band breakup. Later on, Twilight brings the bands back together and becomes the new leader.
From the first clip, I can see Aria Blaze and Sonata Dusk are incompident sidekicks. So in the end, when the Dazzlings get all the power they need, Adagio Dazzle betray her assosietes. Probably taking their magic necklaces and all the magic they collection. So at the end when Adagio is defeated, Aria and Sonata are redeemed and are accepted by Sunset Shimmer as friends, because she understands what they went through.
And speaking of Sunset Shimmer, in the second clip, everyone doesn't like her after what she did in the first movie. But in the caferine scene, the Dazzling turned everyone against each other, wantng to win the Battle of the Bands. They start acting like the old Sunset Shimmer from the last film. So at the end of the movie, they forgive her after walking a mile in her boots.