I wonder if Frontier would consider either ILG-PIT, or TTN-PIT. There is really no low fare carrier on PHL-PIT since Southwest pulled out. Just exorbitant fares on US. Amtrak takes 7 hours and MegaBus is the really only other option but it's long. 276 is fine for those in the suburbs but not great for Center City residents that have to trek to Valley Forge before getting on the Turnpike, and not great for So. NJ either to Pittsburgh region.

Southwest was flying 4x daily trying to capture business traffic and volume traffic. That's a bit excessive, but 1x daily between PIT and ILG or TTN might work for volume and some business traffic.

I don't know if Frontier flies routes that are about 250 miles however, although it seems so from DEN. Their sweet spot for short haul might be 350 miles (e.g. TTN-CMH) even though the demand for CMH/Columbus, OH is questionable to me, while demand between the Philly region to PIT/Pittsburgh, PA is likely a lot higher.