With the pull-down in transcon flying and the drastic reduction in fleet size (transcons will eat up a plane for most of the day), I don't see any significant expansion to those markets.
I think PDX has the best shot, at 1x or 2x daily from PHL, perhaps a redeye and a flight to connect with the European bank. SNA has been done before, and could work, but there are slot issues there. YVR could work seasonally, but it uses too much airplane for this small fleet; a weekend flight would work I think. SJC/SLC/SMF I think are all too thin to work from PHL. I think they're all too thin to work from CLT seeing as how HP wants to focus the CLT transcons on O&D only, even slashing service to SEA and SAN.
Now where I think US will be seeing lots of new service is in midwest markets that can be reached by the new CRJ-900s coming over from Mesa, or other RJs already on the property. Markets like OMA, AUS, SAT. I could see OMA at 1x daily to PHL and 2x to CLT, AUS and SAT at 2x to CLT each.