More jf-17s can't cost that much, plus already 300 jf 17 thunders are more than enough. 250 was original order with 50 as backup, hence 300 are already allocated for. Hence it's not for jf-17.
J-10b was scrapped by senior PAF officials as it didn't bring that much a change from f-16s , PAF is looking to invest in something better than f-16s for medium category fighters, hence j-10b for PAF is out of question unless we need them immediately for stop gap purposes which is highly unlikely.
Chinese Flanders are a no no. Flankers of chinese origin have never been on export market or export exhibitions. Hence very less chance of them being sold to pakistan. Although Chinese officials have said in recent past, even though many weapons are not for export and are secret from the outside countries. There is one brotherly country Pakistan, which has and can have access to weapon systems in chinese inventory which are not cleared for export. China can transfer units already in use in armed forces to the only brotherly country provided there is urgency. But realistically this 1.5 billion USD doesn't seem to be for chinese Flankers either, plus most likely pakistan will not opt for chinese flankers, multiple reasons for it.
J-31 option is very very likely as 1.5 USD billion is exactly the amount that would be needed for 40+ stealth aircraft plus it's support , training and startup ammunition.
If the platform proves itself, this bird might be the second jointly produced fighter among Pakistan and china.
Russian SU-35 or mig-35 are highly unlikely as India gets red faces if pakistan gets these assets , which Russia cannot afford. But it's entirely up to russia, if they can than PAF would easily go for SU-35 along with j-31.
Joint venture with Turkey and BAE systems in TFX is also very unlikely at this stage. As it's very early. But 1.5 billion would be enough to get stakes in Turkish TFX programme.
2nd hand F-16s from USA or third parties like Europe or middle East? Please no! Even though we were in love with f-16 May be even now are. That's not a smart platform to invest 1.5 billion dollars in. We should be moving away from sanction prone assets.
If we have learnt anything from our history.
S-400 triumph missile defence system? May be if, PAF controls and procures missile defence systems. 1.5 billion USD per year would be good enough save in 3 years we could see s-400 in pakistan. As it's a defensive weapon and Russia has said it has no problem selling it to, pakistan , Turkey or Indonesia.