The final scene in The Avengers was the reveal of the mad titan, Thanos. It caused speculation to run wild on when Marvel’s big bad would wreck havoc in the film universe. First it was thought he would be the antagonist in The Avengers 2, but then Marvel Studios revealed their “Phase 2” of movies.
It didn’t take long for rumors to crop up that Thanos would play a huge role in Guardians of the Galaxy. The sole purpose of one of the heroes is to destroy the mad titan. It was a safe assumption.
Marvel Studios’ golden boy Joss Whedon shined some light on the rumors about Thanos’ involvement in future Marvel films.
“Thanos is more powerful. He’s not someone you just trot out and punch him. Like you did in the comics, you want him to be threading through the universe and to save the big finale for the big finale. He’s definitely a part of what I’ve got going on.”
In Marvel comics, Thanos is pretty much the endgame; the baddie to end all baddies. Throwing him in as the main villain in a “Phase 2” movie would be jumping the gun. It’d be a lot like having the crescendo in the middle of a song.
Of course this means Thanos could make an appearance in Guardians of the Galaxy or The Avengers 2, but not as the main antagonist. Although this means rumors will now point at “Phase 3” for Thanos hitting the forefront and putting the boots to the heroes.