Marvel's Kevin Feige confirmed to IGN (via TotalFilm) that indeed cosmic villain Thanos (Vin Diesel?) will play a big role in Guardians of The Galaxy afterall (rumored for Avengers 3, not the second film). Actor Lee Pace (Hobbit Trilogy) has been confirmed for Kree leader Ronan The Accuser (leading into Inhumans?) and Doctor Who actress Karen Gillan will be playing Marvel villain Nebula. New addition Djimon Hounsou will be playing Kree warrior Korath The Pursuer in the film as well. This information comes straight from the Comic-Con panel. Including a bunch of prop photos that feature Starlord's helmet and the Nova Corps uniform. James Gunn also mentioned that casting announcements for Rocket Raccoon and Groot will be very soon.
“I think somebody on the Guardians panel let loose that Thanos is a part of the movie, which he is,” revealed Feige. “Thanos plays a part in Guardians, as the mastermind.”