According to slightly distinct but obviously overlapping reports at Latino Review and The Wrap, Josh Brolin will be providing the voice for Thanos in the Marvel movieverse and we can expect to see him in Guardians of the Galaxy, Age of Ultron and whatever the third Avengers movie is. (Something with the word Infinity in it, I'm sure.)
The Wrap say that his role in Guardians is smaller than his role in Ultron, which is interesting as his role in Guardians is certainly much, much bigger than the cameo he made at the end of The Avengers. I say that definitively and confidently based on observations made on the set of Guardians of the Galaxy.
It's a very strong piece of casting, and I hope Brolin does his best with it. Of course, he's been in an adaptation of a
Marvel comic before and… oh… I even regret bringing it up. Anyway. Onwards and upwards.