Since super hornets are still being produced for USN in large numbers and will be around until probably 2050, I think J-15 production will be ongoing for a while. Big question in my head is what special missions variants could be produced for J-15.
One obvious answer seems to be a buddy refueling aircraft. A J-15 aircraft would already have greater internal fuel than MQ-25 and just a little under combined internal/external fuel of super hornet. The latter is widely used for buddy refueling with USN carrier. We've already seen J-15 doing to buddy refueling.
I would say that a J-15 with 4 external fuel tanks would be able to haul a good amount of gas. If they want to increase tempo and endurance of their air wing, it would seem to me that adding external fuel tanks to J-15s would be a good start. They could also try conformal fuel tank, but that would take more development cycles to accomplish. On the flip side, it would be far less development cycle than a refueling UAV like MQ-25. If J-15 can carry a lot of fuel through the addition of conformal and external fuel tanks, that would significantly reduce the number of sorties it would need to make. A naval aircraft takes a lot of pounding on each take off/landing off a carrier. You would to reduce that when possible by carrying more fuel per sortie.
J-15D is already been development. I'm curious to see if the current one in development will be strengthened in the same way as J-15B to handle the additional air frame stress of catapult. Maybe the actual one going into production with be J-15BD. Since they've already done the work of a two seater with J-15D, they can do additional variants based on that. As we look into future, dedicated AEWC&C will become increasingly less survivable against long range missiles and stealth fighter jet. I think networking, sensor fusion and mini-AWACS are more likely the trend of the future. As such, why not have a mini-AWAC version of J-15 two seater? SAC can add additional radar antennas and passive detectors around a two-seated J-15B with an uprated engine. Such an aircraft would have better 360 degree coverage and would still have the agility to be far more survivable than KJ-600 of Z-18s.
A two seater J-15B with external fuel tanks and uprated engine would also be a good missile truck for anti-ship and ground attack missions. I think there are a lot of possible roles that J-15s can perform into the future that you probably do not want a J-35 to do.