Do not think PAF would go after more F-16's because the delivery would take them 5-7 years hence by that time the F-16's would have ex hosted the life of MLU. Now the F-16-blk 50/52 would also be requiring MLU. PAF would be interested in 5th Gen fighter by that time.
If PAF is offered F-35's then PAF might consider them as to replace some but not in very large numbers. However if the TFX is mature enough then PAF would choose it over the F-35's.
As far as Chinese J-31's or some other is concerned would also make it in PAF as per the policy of PAF it tends to rely on various sources rather than just one. Russia might also offer the PAK-FA or PAK-MA by that time, however would that deal would be availed is to be seen once PAF has inducted Russian Aircraft.
A stealth single engine aircraft would really be interesting because that aircraft would give PAF a low cost second line of defense... Which would this be is going to be finalized once conformation is received if such an option is available form China, Russia, USA, Sweden or a Pakistani aircraft.
JF-17 would not be enhanced to a stealth aircraft but a completely new aircraft would be manufactured so the cost would really be taken into consideration.
To spice up the competition may be the Turkish single engine design might be considered but that too is too early to say if that could be possibility or not.