My opinion is that NJT/MNCR is doing itself a disservice by not capitalizing on the large amounts of people who want to spend an evening in the city, I mean it's the "City that never sleeps" yet you've gotta be out of Manhattan by 7:30 to catch the last express. 64 and 66 are very well timed for this purpose, and they can easily get another inbound by making the return off 49 into a revenue trip. My take is that 67 primarily exists to serve as a Main Line local and they had a set that they needed to get out to Port Jervis, so they just let it take passengers instead of running empty. BTW, weekend train 1739 actually continues to Port Jervis as a deadhead on Sunday evening's according to an employee timetable I got my hands on recently. The same timetable also shows a deadhead X52 on Saturdays that runs from Port Jervis to Hoboken. Figuring this was/is a way to get a set into Hoboken for the weekend should it need maintenance.
Since the local section of 67 has plenty of ridership, having it run as it's own train would allow 67 to be express. Guessing it would need a deadhead return to Hoboken, or just run revenue but primarily just to get the train back. It could also deadhead to Ridgewood and take 68's place on the Bergen Line and actually make all the stops.
While just having 67 be an express would be a big improvement for Port Jervis Line riders, to really get the late evening service right would require an additional train. In this case, 67 would need to be moved up so it would meet 68 either at Harriman or between Sloatsburg and Suffern (instead of at Route 17). A new train departing Hoboken at around 11-11:30 could be well utilized by those leaving Broadway shows or events ending at similar times. To get the equipment for this train, I'm guessing 63 or 65 would change to NJT equipment (like 59-68) and deadhead back to Hoboken following train 68 down. This would free up a Metro North set to run on the new train, which would finish for the night upon arrival at Port Jervis. 41 would likely remain unchanged.
Implementing this service would require an extra Suffern round trip and an extra Port Jervis round trip. Not sure what the cost of that would be, or the additional revenue of people switching to the train who would have used other means of transportation both ways before.