Creditors of American Airlines, now in bankruptcy, reportedly are pushing for US Airways to take it over. It kind of makes sense. The two don't have a lot of overlap. The merged Delta/Northwest and United/Continental have merger growing pains and such a new merger wouldn't pass antitrust.
Beyond that, Southwest surely wants nothing to do with American; likely, the same is true for Jet Blue. Would an off-the-wall mid-sized carrier like Alaska Air venture in? What about a foreign airline, but one not owned by a foreign government? Would the U.S. government waive regulations?