Though United will most likely remain the largest airline in the United States and the world for some time, there is no doubt in my mind that American will still be a dangerously close second. American will receive quite a few perks by taking over TWA.
America will erase competition in San Juan, which had somewhat recently become a TWA focus city. With an approved merger, there will be no stopping AA in dominating the Carribean.
American Airlines will delete TWA in JFK, TWA's second largest hub. American also has big plans for JFK, and their merger with TWA will undoubtedly speed up the process as well as make it easier.
American gains TWA's focus city in Los Angeles, including TWA's recently aquired LAX-DCA flight. This, along with AA's aquisition of Reno Air will finally give them their long-coveted West Coast market.
American gains STL. With St. Louis in American's list of hubs, the airline will own the Midwest. Their hubs wil go up and down the entire area of the country from ORD to STL to DFW.
American is the definite winner in this merger, I don't see how it can work any better for them.