From PR Newswire
"[A] new dark force threatens Ponyville, and the Mane 6 – Twilight Sparkle, Applejack, Rainbow Dash, Pinkie Pie, Fluttershy and Rarity – embark on an unforgettable journey beyond Equestria where they meet new friends and exciting challenges on a quest to use the magic of friendship and save their home."

Deeper dive into emotional stakes and character moments: We think the moments are there and the emotional story tracks but our biggest overall note is that we want to really dive in and make the moments much deeper, take the time to build out some of the relationships and really push to make Twilights emotional growth as clear and strong as possible.

Villain arc/motivation: We don’t think Cosmos is quite there yet as the villain. His motivation is unclear and his personality could use a pass to make him truly stand out.

While I am going make some sort of a prediction here base on the describtion about the movie here.
Something going to happen in season final of season 6 that would give the big revealed of or movie main villian Cosmos. Cosmos is an artificial Alicorn created by Starswirl the Bearded he created him be prefect and represent him as the leader of new of three tribe coming together. But when word got out that there was an alicorn community out there which made so scare he is not prefect anymore he decided to create the dark force that ended up destroying the alicorn community and killing anyone in it except Celestia and Luna. Starswirl found out what Cosmos did and decided to imprision him for the time being and ended up raising Celestia and Luna. Almost 1000 years later he was free and decided unleash the same dark force that he send on the alicorn community.