I have to admit, The Avengers movie is certainly something that will be a sight to see whether it is a good movie or not. Seeing Iron Man, Captain America and Thor onscreen together for the first time, and possibly even the only time (I doubt but you never know) will single handedly make it a crown jewel.
But I have been trying to figure out something, who in the name of Odin could Earth's Mightiest Heroes face off with? It seems impossible really to have just one villain face off against a team like The Avengers, so I am trying to figure this out, and I think I am preaching to the choir here.
Loki would seem like a great villain, he was the first foe in the comic books that The Avengers had to face off against. In the comics, he took control of the Hulk's mind and turned him into a threat against the team then consisting of Iron Man, Thor, Ant-Man and Wasp. Using Loki would be beneficial, as far as thinking about how the character will already be setup in "Thor", and a logical reason of why Thor THE GOD OF THUNDER would join a team of mortals: Help to fight against his brother.
That way Loki and Hulk would work as villains, which would be perfect as far as im concerned. However, rumors have persisted in the past that The Skrulls would be the big threat that Nick Fury has been aware of since "Iron Man" and would need to form a team to fight them off when they invade the Earth. Which makes sense, and certainly it would be a little weird for that NOT to be the big threat since Nick Fury obviously has a reason for assembling The Avengers, and Loki and Hulk is not something that he would know of really, and even if so, why assemble a team for that?
Personally, I beleive it will be one of those couple scenarios. But if not, I think a good villain could be Graviton, who was a villain in the first episode of "The Avengers: Earth's Mightiest Heroes". The character showed off enough for me to beleive he would make a cool villain visually, but the only thing I dont know of is if he would really matter at all to a live-action film.
Could the villain be created for the movie? That could work maybe, but with so many villains already existed it would seem very unecessary.