I am pretty sure the movie is dealing with the Negative Zone. Think about it. Where else would Loki have been transported to after he fell through a wormhole? Look at this still. Doesn't it look like a new dimension is being opened?
Anyone who is familiar with the Avengers have heard of a particular villain known as Annihilus who is a lord of the Negative Zone. He once invaded Asgard and was especially featured in a 2006 series called Annihilation Wave. Here are some of their alien space ships:
Notice the group of Annihilation Wave space ships destroying its enemy fleet. Look at the ones in the upper right hand area which are rather fish-like.
Now look at these pictures of the "Leviathan" from the trailer.
Don't they look similar?
Now look at Loki in his suit during the Stuttgart, Germany scene where he seeks to subjugate the crowd. Look at the "staff" and the flashing gem he's wielding:
People thought he was using the Cosmic Cube attached to a staff....but what if it is the Cosmic Control Rod? Here is is a picture of the Cosmic Control Rod chained around Annihilus' neck.
Here is the Cosmic Control Rod as its glowing by itself:
Notice the similarity in shape to the tube like gem attached to Loki's staff.
My theory is that Loki fell into a wormhole at the end of Thor where he enters the Negative Zone and encounters Annihilus. There he makes a pact with the Negative Zone warlord to bring the Annihilation Wave to Earth. Annihilus then gives Loki the Cosmic Control Rod to help accomplish the task. Here is where the Cosmic Cube comes into play: Loki will use it to open up a direct portal to Earth and fights the Avengers with the Rod.
PS: A heads up from Tainted87 pointed out more proof that Annihilus and the Annihilation Wave will indeed be the other main villains in the Avengers backing up Loki. Right click on this wallpaper image from the Avengers Movie site. Now left click on "Open Image in New Tab". Look at the URL:
What does it say?
"http://i.annihil.us"
Case solved!