According to Cinema Blend on Tuesday, a movie spoiler finally revealed Thanos' role in James Gunn's "Guardians of the Galaxy." Currently, James Brolin has been casted to do the voice of the Mad Titan, but now sources reveal his more expanded role in the film that ties into the Marvel Cinematic Universe.
The situation here entails the Mad Titan appearing in the first act of the film as a hologram communicating via "Star Wars" style with both Ronan the Accuser, played by Lee Pace, and Nebula, as portrayed by "Doctor Who's" Karen Gillan. So it looks like he won't be "live and in the purple flesh" as some were hoping, but it does a provide a good build-up as the Marvel movies make their progression. There also looks to be some dialogue from the character and also is mentioned in a fearful fashion from characters throughout the movie and also has family relations with Zoe Saldan's Gamora character. So it appears to be comic book accurate in that regards.
Sources have indicated that Thanos won't be making his appearance in Joss Whedon's "Avengers: Age of Ultron" to either team up or have any involvement with their foe. So it looks like at that point, Thanos will holding off until further notice and making his debut in "Avengers 3" where he would likely be the main adversary that would likely carry out the events which lead up to the Mad Titan's acquisition of the Infinity Gauntlet and the corresponding Infinity Stones.
There has been so much build up in the Marvel Cinematic Universe that by the time Thanos makes his appearance, it's likely Marvel Studios would have to considerably plan a momentous occasion as Earth's mightiest heroes oppose a force equal of that to a god.
"Guardians of the Galaxy" opens in theaters come Aug. 1 and "Avengers: Age of Ultron" is scheduled to follow there after come May 1, 2015.