With the release of a bunch of characters over the past few days there has been a fair amount of speculation as to what the movie will exactly be about. Over on Derpibooru someone found an article about the movie on the Polish site fdb where a possible synopsis has popped up.
As of now it is just speculation, not confirmed, at least until we hear the same thing from some other sources.
Discuss it down in the comments! We'll let you guys know when we find out more. Check after the break for the translation and sources for those of you avoiding spoilers.
Thanks to Stefano and Maik for sending it in and Vengir for the original translation.
Update: One thing Vengir pointed out in the comments was that this article was published six months ago, before the reveals of the characters in question. So they couldn't have come up with the synopsis from the character reveals it seems. The plot thickens!
Update 2: Another Polish website has the same synopsis according to Stapart -
Indeed it's true (probably), on the website www.filmweb.pl/ (http://www.filmweb.pl/film/My+... ) biggest Polish website for movie and TV series reviews, appeared exactly the same synopsis (with a description: "a description of the distributor cinema").
Update 3: Third source provided by mrx1983 here.
Synopsis from polish website reveals some more plot details:
The herd of brave ponies comes eye to eye with their biggest enemy. The mischievious Storm King decides to steal all of the magic from their home Kingdom of Equestria. In order to save it, the brave heroes have to set out to a journey full of adventures. They will make new friends like seaponies or good pirates and will live through many emotional and funny turn of events to once again prove that friendship is magic.