There are two possible options J31 as popular choice of the forum and the j20 which I think would be game changer.
J31 is the optimal addition if the PAF mandate of denying air superiority to India remains the same.
If the mandate of the PAF changes, which is the aspirations of most Pakistanis where PAF would want to take the fight away from it's airspace into the Indian airspace, then J31 doesn't fit the bill, let me propose the J20 platform would be a more potent albeit slightly more expensive system or a CFT variant of the J31 with extended range.
Engine: The J31's use the same Klimov RD 93 engines as of now on the test bed which may or may not be replaced by a chinese variant. Compare the J31 ranges with the Mig 35/ Mig29 OVT and Mig 29 K which uses the most recent variant of the RD33 series - RD33 MK, similar 17.5 Ton loaded/Gross weight, the ferry range of the Mig 35 is 3100Km with 3 Ext Drop tanks, whereas the J31 is being advertised as a 4000 Km Ferry range.Given that both aircrafts are of similar empty weight around 11-11.5 tons and have a 17.5 ton loaded weight, there is a good chance that both carry similar internal fuel, thus the combat range of the J31 in stealth configuration might not be1200 Km as advertised, rather around 500-600 Kms similar to that of a F16. That would put the idea of deep strike into a pickle.
Airframe: AVIC says that a lot of additive mfg was used in the test bed, and hence could not be dis-assembled during the transportation for testing. As the production models are out to 2019, if any of the RP techniques make it into the production model it will be a nightmare of maintenance and fitment. In addition the airframe was reported to bleed a lot of energy in it's first appearance, hopefully by the production variant those issues might be resolved.
Source:
https://defence.pk/threads/paf-the-...les-sale-to-india.427156/page-2#ixzz4PzrZTJXG