Nearly a year ago, a rumor popped up that the main villains in the upcoming Avengers movie would be none other than the Skrulls. Now, Latino Review has apparent confirmation that the villains will indeed be the Skrulls and Loki will also factor into the plot along with the Cosmic Cube. Loki being involved ties in with the very first Avengers story, where he brainwashes the Hulk, and putting the Cosmic Cube in there ties together the recent Marvel movies.
The Avengers is what Marvel has been building towards ever since they founded Marvel Studios. It’s their goal to have a cinematic universe the same way the comics inhabit the same universe. So far Captain America’s shield has appeared in both Iron Man movies, and it makes sense as Tony Stark’s father Howard Stark creates the shield in the upcoming Captain America movie. In addition to Tony Stark showing up at the end of The Incredible Hulk, Tony is seen viewing the aftermath of the college fight from Hulk on the video screens at the end of Iron Man 2 (which means Iron Man 2 takes place at the same time as the events in The Incredible Hulk).
The Cosmic Cube is a big plot point in Captain America, and Loki is naturally a big part of Thor, so in addition to bringing all the characters together; The Avengers looks to tie together the Marvel cinematic universe in many ways.