The role being discussed is Peter Quill, who becomes Star-Lord and is the leader of the Guardians. So far it seems like Marvel wants to cast Quill first and then build a supporting cast of heroes around him. In addition to Gordon-Levitt and Krasinski, the current list of actors being considered includes Joel Edgerton, Jack Huston, Jim Sturgess, Lee Pace, Eddie Redmayne, Garrett Hedlund, James Marsden, Sullivan Stapleton, and Zachary Levi.
Moviehole also reported that a former DC actor auditioned for the role. Check out that tweet below.