Rumor has it that Robert Downey Jr and Matt Ruffalo are shooting scenes and it's a post-credits scene that deals with the fallout from the death of a major character — and his rebirth as a familiar Marvel character, specifically the android Vision.  [Comic Book Movie]

The most commonly reported version is that Thanos shows up to reflect on Loki's failed invasion of Earth, and he tells another alien that he will succeed where the Asgardian failed, with the help of the Infinity Gauntlet.
Set up for Avengers 2? [Comic Book]

Catch a clip below: