So the problem with any UA move back into JFK is not only well timed slots, but also gate space.
JFK is becoming an airport with basically 4 terminal in the JFK redevelopment: DL terminal, B6 terminal, AA terminal and international terminal. I thought a B6/UA partnership would have made the most sense for a UA re-entry into JFK, but that clearly did not happen. UA is not going to be able to fly out of the first 3. A read on the JFK redevelopment.https://ny.curbed.com/2018/10/4/1793702 ... renderings
International terminal is going to be very crowded with the 4 primary tenant + most *A carrier + skyteam carrier + a couple of OW carrier and unaffiliated international carriers. And there is only going to be 22 or 23 gates there from what I can remember. There will be times during the day where new T-1 will be completely full. If you think about someone like QR or ET blocking a gate with a widebody for a couple of hours waiting for departure, you'd see what kind of stuff UA would be competing against here.
Keep in mind, T-1 will be charging a lot of money for those gate and space access. The project is apparently going to cost $7.4 billion to complete. All that money will be collected with very high fees. Which makes sense when you have large widebodies. It makes less sense when you are loading RJ or small narrowbody mainline in there.
Of course, it will be a while before we get the new T-1. But the current T-1 is really overcrowded and in bad condition. T-7 has space, but JetBlue is kicking BA and other tenants out of there by 2023 to start its own construction of the new North terminal. That's not even a medium term solution.