Regardless of who runs the show...
The only other possible hub in the Southeast, CLT, where UA has minimal presence and US is the only real competitior to DL's mega ATL, is not going anywhere. Anyone who says otherwise is a moron.
Headquarters would likely remain in ORD. Training centers in ORD DEN PHX and CLT may be maintained due to sheer size of the airline.
All PIT and PHX operations centers etc will be gone.
LAS, done. No need for a low yield leisure hub, unless they want to continue the nighttime hub idea for aircraft utilization. In these times though the planes may be better off on the ground.
PHX and DEN and PHL and IAD are the two tricky ones. One of each will become an O&D focus city while the other remains a connecting hub.
If able to do so, I'd keep DCA and IAD as a high O&D dual presence- Shuttle and east markets from DCA, transcon and international from IAD. PHL's geography and more established routes are better for a Northeast connecting hub, with larger aircraft and less frequency. Parking the massive amount of rat jets clogging up the corridor will be a huge help. Making better use of LGA and BOS slots with larger aircraft and more important destinations would help also.
DEN and PHX is a toughie too. PHX is infested by WN while DEN has weakened F9. DEN is better located as a hub and a much better facility.
The question of Express carriers is interesting too, as that's where most of the capacity reduction will and should come from...
Piedmont (owned by US Airways)
PSA (owned US Airways)
Air Wisconsin (US)
Mesa/Freedom/Air Midwest (US and UA)
Chautauqua/Republic/Shuttle America (US and UA)
Colgan Air (US and UA)
Trans States Airlines/GoJet (US and UA)
SkyWest (UA)
Reductions in ground staff at affected hubs are likely, but I don't think there will be huge reductions in mainline flying, at least not to the degree US Airways east employees saw when almost 50% of some work groups were laid off.
I'm just speculating as to what makes sense, not sticking my tongue out at a percieved enemy like some of the above posts... Keep in mind some pieces could be sold to others. Also keep in mind nothing has been announced yet.