I am guessing American Airlines or Jet Blue would be the prime targets to take the shuttle. More so American Airlines because it would create more competition between itself and Delta for frequent flyer members. There is always a wild card though, US Airways could spawn the Shuttle into more markets and just go by US Airways Shuttle or something, and eliminate everything else including it's hubs and carry a fleet of regional jets and a few Airbuses to feed the Shuttle from major cities only. This would create small hubs for itself in DCA, LGA, BOS, PHL and perhaps a new shuttle market like FLL, JAX, TPA, MCO. That would suck wouldn't it?
As for Delta buying all of US Airways, I would think that would be very doubtful. How full is ATL? I just don't think it would make sense to split some of ATL to mix it with CLT. The two hubs are only about 300 miles apart.
Someone has to take up the slack in CLT. Currently there is over 500 departures a day out of there from US Airways and US Airways Express alone. That is unless it went from the current hub structure type of airport to a more generalized airport with a strong mix of airlines like RDU.
With a large maintenance facility, parallel runways, large modern terminals for domestic, International, and regional jet traffic, a baggage system to handle a massive amount of traffic from one airline, and being positioned smack in the middle of the east coast with limited weather problems, it's got to be a valuable asset for a major airline to continue using CLT as a hub. With Delta's and Air Tran's hub so close in ATL, and Northwest just over the mountains in MEM they are unlikely carriers to be interested in taking over the CLT hub. That leaves us with United, Continental, Southwest, and American. Southwest doesn't believe in hubs, and are doing well in RDU, so they are very unlikely. Would American abandon it's new efforts at RDU for CLT? What about eliminating a current hub to get CLT such as STL? Reduce traffic in MIA and mix it with CLT? To me, that would be a unnecessary effort. So that leaves just United and Continental. Is United in a position to take on more aircraft, and a entire new hub. I know they expressed strong interest in CLT during the merger talks with US Airways. After September 11th however, they are not far behind US Airways financially. I am sure they would take it if they could, but can they, and what about Washington Dulles? So that just leaves Continental. They have a strong hub in the Northeast, EWR. They have a strong hub in Texas, but nothing on the southeast. They tried with Continental Lite in GSO and failed, but that was Continental Lite, and still a different Continental all together. I think a Southeast hub would fit them nicely, although it's a toss up between the two in my opinion. Continental would most likely need more planes, to handle another major hub, and I don't think they would like all those Airbus. So maybe United does have the advantage, but I am sure they are pulling for US Airways to last a little longer so United can recover themselves.