First of all, I think the new F9 ownership is setting them up to merge with either Spirit or Allegiant, so those two will not be in the running.
I think the best candidate would be Delta, as unlike United and AA/US Air, it does not have a hub nearby from which TTN would take traffic.
I think a legacy carrier with flights (2x/day) to a midwest hub (likely DTW in DL's case) would be the way to go.
I think it would be with a regional, with a jet in the 70-90 seat range (CRJ-700 or -900 or the E-170/-175/-190).
This would have one carrier (F9) focusing on leisure/VFR, while the legacy carrier could focus on business with connections.