A DL/CO merger would perhpas consolidate two mature East Coast/Latin America/Europe operations and result in a strong airline focused on developing in Asia and the American West.
I think that the DL/CO hub structure in New York is complimentary. Delta uses JFK primarily for O/D traffic, with a limited number of feeder flights to its connecting hubs and larger domestic markets, and it's West Coast cities. Continental has developed Newark as a true connecting hub, pulling in feed from all over the country. A combined DL/CO could easily maintain lucrative O/D operations at JFK and international connecting opps at EWR. Say what you will about monorails, airtrains, and the like, most New Yorkers (myself included) vastly prefer a trip to JFK than to EWR -- given the prevailing auto traffic conditions, Manhattan's West Side is really the only part of the city more convenient to EWR. Besides, Jersey is anathama to Manhattanites! For this reason it would be wise to focus O/D opps at JFK, and connecting (and New Jersey O/D) at EWR. Coast-to-coast flying would be profitable at both airports, and the combined CO/DL could reserve LGA for the VERY lucrative business market (to Boston, Washington, Atlanta, Chicago, Philly, Miami, Detroit, Dallas etc . . .). Most of these markets have their own European service or are closer to other international gateways, and hence don't need to feed into JFK for international flights. As for the New York's Latin America service, most of that is O/D as well, as people elsewhere in the country connect through Miami, Houston, or Atlanta.
Which brings me to the other opperating efficiencies of a combined DL/CO: Houston is a goldmine, and Delta could basically pull out of Dallas in favor of this little gem. Cleveland would give way to Cincinnati, of course. The midwest is the undisputed champion for connecting traffic (almost major has a hub in the midwest: ORD, DTW, CLE, CVG, STL). Chicago, like JFK and, to a certain extent, ATL, is becoming more and more of an O/D market. Besides, AA and UA already duke it out there. Detroit is, to phrase it politely, less than desireable, and in the collective American unconscious is synonymous with a trip to hell. (Once NW is bought out, it will probably see its opps reduced). Long and short, CVG is the best-positioned midwest hub, and Delta's opps there are a license to mint money.
Shoring up these operations would leave the combined carrier a strong position to develop Asia and West Coast service. I have long believed that Delta could become the dominant East Coast carrier to Asia -- what with increasingly sophisticated planes and growing markets, direct service from New York (and Atlanta) to major Asian destinations is entirely feasible. There is no really convenient way to get to Asia (with the exception of Tokyo) from the East -- most flights must connect through Detroit or LAX/SFO.
Perhaps DL/CO could also build up at LAX (DEN? SFO?) and could pull down SLC -- yields there aren't too profitable.
Your thoughts . . . and apologies for the stream of consciousness style of this post!