Loki

The first villain everybody thinks about is Loki "“ as said before, this is one of the most complete characters in the MCU, always scheming and always searching for a way to defeat his step-brother and take over Asgard. At the end of Thor: The Dark World he apparently succeeded "“ after helping his brother defeat Malekith the Accursed, he faked his death and somehow managed to overthrow Odin.

But what is his endgame? Almost everybody believes that he is still under the influence of the Mad Titan "“ we actually saw fear in his eyes, in the first Avengers movie, when the Other mentioned his master. So it isn't hard to see him clearing the way for the arrival of Thanos. And Thor: Ragnarok might just be the movie in which his story arc becomes completed "“ since this is Disney, we can actually bet that Loki will, in the end, find redemption and somehow come to the aid of his step-brother (losing his life, for real this time, in the process).

But things aren't as easy as they seem. As we all know, Thor: Ragnarok will feature one other Avenger: the Hulk. We have no idea, yet, how he reaches Asgard. To have the God of Thunder bring the Jade Giant as an ally seems too weird a thing to do, especially since Hulk isn't such a stable character. Instead, we believe that Loki will manipulate somehow the Green Goliath and mind-control him into defeating the entire Asgard. Just like in the awesome Hulk vs. (you must have seen this coming a mile away for Thor: Ragnarok)

But we also believe that, in the end, Loki will see the error of his ways and release the Hulk from his grip. And this is where Thanos will enter, beat Thor to a pulp, and throw the Green Goliath like a ragdoll.