My best guesses for a TRN-like focus city would be BED, GYY, or FTW. They all share common Trenton traits - no current service to them but existing terminal facilities in place and a large population base where they would be the closest commercial airport, plus additional people closer to other airports but could be stimulated by low fares.

IAG and PBG have ULCC service, are not the closest airport to a large population base, and are really only doable to major tourist destinations like Florida/Vegas, which is already well served by Allegiant/Spirit.

HVN might get a couple cities, but as you mentioned, the runway may be too short for longer flights. No Florida, no Chicago, no Atlanta.