CCR (Buchanan Field, Concord, CA) used to have service from PSA and, if I recall correctly, USAir. That was well before the population of Contra Costa County and environs swelled.
Longest runway is 5,000 feet, so I am not sure if that is adequate for a fully loaded 737. I think PSA flew BAE146s in and out of CCR.
My dream airport for Southwest service is Astoria, Oregon.