Although we saw Thanos at the end of Marvel's The Avengers, we haven't seen him since and he hasn't been featured in the two full trailers for James Gunn's Guardians of the Galaxy. Gunn has previously revealed that the August 1 release is connected to Thanos, however.
"An article on James Gunn's post-production process reported a scene with Thanos on a rocket throne in Act 3, which suggests an appearance long enough that someone is going to have to voice the Mad Titan, someone that could take the part into Avengers 3 as a major villain.
That meant Marvel was looking for an actor of some caliber, someone that could give a performance to a very complex, but very purple-skinned alien villain.
Our sources tell us that person is JOSH BROLIN!!
He'll be providing the voice of Thanos in Guardians of the Galaxy and with a name and reputation like that, it's likely he'll join James Spader and Tom Hiddleston as actors who have tried to kill The Avengers."
Joss Whedon has hinted before that we wouldn't be seeing Thanos take on The Avengers until Phase Three of the Marvel Cinematic Universe:
"Well, Thanos is more powerful. He is so powerful; he is not someone you can just trot out and punch him. Like he did in the comics, you want him threading through the universe and to save the big finale for the big finale. He is definitely a part of what I have got going on. The thing about The Avengers is that they are very powerful but not very stable. So, there will definitely be some people that will shake them up in the next installment."
What do you think about Brolin voicing the character this summer and playing him again in Avengers: Age of Ultron?
Josh Brolin to Play Thanos in Guardians of the Galaxy - SuperHeroHype