Here’s the official statement from Disney:
SEQUEL TO MARVEL’S THE AVENGERS TO HIT THEATERS MAY 1, 2015
The Walt Disney Studios has announced a release date for Marvel Studios’ sequel to the biggest Super Hero blockbuster and third highest grossing film of all time, Marvel’s The Avengers. Joss Whedon returns to write and direct the UNTITLED MARVEL’S AVENGERS SEQUEL set for release May 1, 2015.
Now that we know when it comes out, we know where it falls in the cannon of the Marvel Universe. Before we see another Avengers film, we will see Iron Man 3, then Thor 2, then Captain America 2, and Guardians of the Galaxy on August 1, 2014.
There was also a rumored logline for Guardians of the Galaxy today:
A U.S. pilot who ends up in space in the middle of a universal conflict and goes on the run with futuristic ex-cons who have something everyone want.
I’m going to guess that the “Something” is an infinity Gem. That would incorporate Thanos into that film and set him up to have the Infinity Gauntlet in Avengers 2.