Former Goonie and star of Jonah Hex, Josh Brolin has all but been confirmed as the voice of penultimate Avenger's villain Thanos. Audiences briefly saw Thanos at the end of The Avengers, but you'll get a better look at him in August's Guardians of the Galaxy.
Thanos, will race the Avengers and the Guardians for control over the Six Infinity Gems which make the wielder omnipotent. So far, the only Infinity Gems we know about are the Tesseract from Avengers 1, and The Aether from Thor 2.
Get ready for at least four more MacGuffins.