If the airlines do indeed merge, the Baltimore/Washington hub will probably
be one of the first to go. US Airways reduced operations there considerably,
and only started to rebuilt a schedule there with the introduction of Metrojet.
I think Metrojet will be absorbed into the United Shuttle (formerly the Shuttle
by United) although many routes will be pared.
The Philadelphia hub will remain. Charlotte will probably be United's answer
to Delta's Atlanta hub and if they're smart, they will market it as a less-
crowded, more manageable alternative to ATL. Pittsburgh is a very big hub
(it is, in fact, US Airways' largest, with close to 500 daily flights) so it is most
likely to remain and allow United's extra-capacity to by pass ORD and I
would not be surprised if the carrier is forced to move some flights or some
frequencies from ORD to PIT to alleviate antitrust concerns, not to mention
congestion at ORD.
The Dulles hub could be merged with the PHL one, in which case PHL could
go, although Philadelphia is the fifth largest metropolitan area in the USA
and a viable hub city.
If the merger is approved, United will have to reduce operations at Reagan
National, Boston-Logan, and probably LaGuardia as well to appease the DOJ.