Gordon-Levitt is reportedly being considered to play lead Peter Quill aka Star-Lord, "the son of a human mother and alien father and captain of a psychically-controlled spaceship who serves as the audience’s entry-point into the film’s galactic sprawl of weird and wonderful characters."
In November 2012, speculations that Gordon-Levitt will be the new Batman surfaced, when the last scene in TDKR showed his character Blake discovering Bruce Wayne's Bat Cave.
Here's the catch: Batman is under DC and Guardians of the Galaxy, as mentioned, is under Marvel.
But according to Total Film, that Gordon-Levitt will reprise the Batman role is not yet concrete. So, technically, the actor is free to take on a project from Marvel.
Guardians Of The Galaxy is set for worldwide release on August 2014. It will be directed by James Gunn. - Rappler.com