As a native San Diegan I would have to say PSA. No questions asked. You gotta love those "smile-liners" and you can still to this day find the occasional US Airways aircraft (not counting their PSA heritage livery A319) with the distinctive "smile" painted on the underside of the nose.
Hubs: Keeping it realistic I'd say SAN (of course), SFO and LAX. Nothing to change from the original.
Fleet: Airbus 320 family (short/medium haul) and Airbus 330 family (long-haul)
Location: Duh... the Pacific Southwest! Kidding... HQ in San Diego, all flights in/out of California as the original ran.
Outside of that, I'd love to see Pan Am return in some form. In its heyday Pan Am was what every other airline in the western world longed to be.
Hubs: JFK, MIA, LHR, NRT, LAX and HNL
Fleet: I see a present-day Pan Am heavily using the Boeing 777 (all variants) with B738/B739's for Short/Medium Haul routing
Location: HQ in New York, flying everywhere they can safely put an airplane down and get back out again.