According to The Escapist on Saturday, "Sin City: A Dame To Kill" and "Guardians of the Galaxy's" Josh Brolin who did the voice work in James Gunn's galactic film had revealed of one additional role as Thanos he is aware of. However, the other two he knows nothing about and is pleased to know how much fans and crew are very much into the franchise.
In the past, there had been a rumor going around about him portraying the Mad Titan Thanos for Gunn's film, it's now obviously made apparent. This did not really appease fans enough to see the Nihilistic villain do his dirty work short of just saying to Ronan the Accuser, "Ill pave the star ways with your blood." as a mere threat if he doesn't return with the Infinity Stone.
It is now pretty much understand that his beast of a genocidal maniac will play a huge role in the Marvel Cinematic Universe, however, it has been up to question how many times he'll be making an appearance. Brolin makes his statement as such:
"I know what one appearance is going to be for sure. There's one or two that I don't know."
Further in the interview, Brolin was questioned about how top secret he's been privy to keep certain things under wraps when it comes to the future of Thanos and the Marvel Cinematic Universe. He had been rather vague in his response only because it's likely Marvel prefers to keep it that way. Even Brolin emphasized the secrecy of his role being that the studios just took so long for them to make the revelation apparent at the 2014 San Diego Comic Con. He even mentioned for as long as he's known Robert Downey, Jr., prior to the Comic Con panel, the "Iron Man" actor claimed it to be "a whole different universe."
For those unfamiliar with the Mad Titan, Thanos is a descendant of Utopian-like society contained on the moon of Titan which orbits the planet of Saturn and later wreaks havoc on the universe by catering to the personification of death which is more of a mistress to him by attempting to create a balance in the universe by killing off as much of life as possible to appease his love, Mistress Death. Of course, this is the portrayal of the story line in the comics, however, the Marvel Cinematic Universe may have different ideas.