Ok comic book nerd time...
In the beginning of the Avengers original story (multiple versions, i.e. Ultimate, West Coast, and multiple reorganizations) it was Iron Man, Thor, Ant-Man, Wasp, and Hulk. Loki took control of Hulk and caused all sorts so Crap to go down. Hulk was then sorta kicked out due to no one being able to trust his rage. That is when Captain America popped in to the picture.
Now the rumors I have heard are Cosmic Cube (correct name) for Captain America and then possibly the Infinity Gauntlet for the Avengers (which is also shown in Thor apparently, have yet to see it). Now the possibilities are endless with story line for the big team up. I have heard potential Loki/Hulk mixed with some Skrull or Chitauri from the Ultimate universe. If it is the Chitauri then that leads into a potential Black Panther cameo.
When Marvel took control of their properties they have done an amazing job leading into this. And it seems they have yet to disappoint.
Oh and for those who didn't bother watching both Hulk movies (I know they are pretty bad), you really should they will play a big element probably in what is to come.
Now I need to clean up my mess from thinking about this.