In an interview with Empire Magazine, Marvel Studios president Kevin Feige gave brief details about Thanos' appearance in next summer's Guardians of the Galaxy movie.
"Ronan The Accuser (Lee Pace) is very much the main bad guy. And Thanos is lurking above it all. You will learn more about Thanos in Guardians, for sure. Certainly you’ll get more than the one turn-around-and-smirk. You’ll get much more than that."
Also known is at the start of Guardians of the Galaxy that Ronan, Nebula, Korath and Gamora are all working together and for Thanos. While not much is known about Benicio del Toro's character the Collector, this is a tempting spot with a lack of information from Marvel that deserves some thought and guesses.
Perhaps del Toro is really playing Thanos or maybe fans will have to start assuming that Vin Diesel could actually be Thanos, since he is still not confirmed to really be voicing Groot yet. While Marvel has been making announcements through their own website for Ultron and Rocket Raccoon (voiced by Bradley Cooper), they have neglected to announce who Diesel really is.
The conspiracy theories continue... and look to have moved on to Thanos now.