Oddly enough Airtrain USED to have 15 min frequency all-day service untill 8pm at night.
This had lost them money, so they had to reduce back to 30 minutes in the off-peak. Surprisingly enough, cutting back the off-peak service made a profit for AirTrain after loses for the first few years.
Though in my opinion, assuming AirTrain is still privately owned, I would suggest improved peak services, and extended night time hours untill 11:30pm at the earliest. Both the AirTrain and the State Government would have to jointly fund any extra services.