It looks like the Tirek-like Storm King is going to be the archvillain of the 2017 My Little Pony movie, and the unicorn with the broken horn (Tempest Shadow) is his ally.

The Storm King and Tempest Shadow use their weather control magic to threaten Equestria.

The Mane Six leave Equestria to try to find Princess Skystar in the ocean (who looks like ruler of the Merponies), and the humanoid cat Capper is also an ally as well.

I figured out the basic outline of the movie plot.