I don't exactly recall the schedule of the now-gone Three Rivers, but putting that back would allow a 2nd PGH-NYP schedule and provide an improvement over the once per day they have now.
Another option would be to put the Cardinal back on the schedule it had roughly 30 years ago, generally 12 hours different than todays' schedule. I often boarded at WAS for the 9 or 10PM departure, and it got to CHI about 9PM as I recall. Back then, it was daily, as well. Made all the difference in the world.
But like adding ANY train to the Amtrak system, it takes money, equipment, crew, and, of course, host RR agreement to run an extra train. If the past 20 years or so history regarding host RR 'taking on' an additional Amtrak train, they unconditionally require Amtrak to pay some exorbitant route upgrade costs up front ($20,000,000 and up, typically) before the host RR will consider adding a train. That's how CSX stymied reinstatement of NOL-ORL for the Sunset Limited, UPs' refusal to make the Sunset Limited daily, and the list goes on...
However, these days, I think the biggest hurdle to restoring or adding a train is what is perceived as a lack of equipment. Having to schedule train turnaround times as poorly as 20+ hours for the western trains at CHI and nightmare NYP turnaround times for most of its LD departures is a crime, in my book. But then, LD trains 4-6 hours late at their endpoints way too often explains the 'need' for such poor turnaround times. The reality is that Amtrak is not the AT&SF or NYC of 60-70 years ago where passenger trains ran mostly on time.