My thoughts.
"Midget" sized submarines are a very good option for our friends. Iran has midget sized submarines called Ghadir, but I think something larger, like the new Iranian submarine called Fateh will suit Pakistan needs nicely.
Fateh submarine has a displacement of 600 tons, designed for the Persian Gulf and sea of Oman.
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Next generation of these submarines will be a larger and utilise AIP.
Fateh is "equipped with sonars, electric drive, combined battle management, surface-to-surface guided missile guidance, torpedo guidance, electronic and telecommunication warfare, secure and integrated telecommunication systems etc. Fateh submarine has speed of 11 knots (20.35 km/h) and is capable of travelling submerged at 14 knots (25.9 km/h). The submarine is armed with four 533-mm torpedoes. It can carry eight sea mines and two reserve torpedoes." The submarine can also fire cruise missiles. This submarine actually has 11 different sonar systems! For it's size, it is extremely well equipped sensor wise and packs a nasty bite!
Some pics from inside:
I am not saying Pakistan needs to go for this specific Iranian submarine, but I think people generally write off smaller sized submarines and focus more on giant, nuclear powered systems. These smaller submarines are a nightmare even for the most advanced navies. A dozen or so of these submarines will give the enemies of Pakistan sleepless nights. I am not sure if this new is related to Pakistan however, it could easily be another nation such as Iraq, Syria, Venezuela etc. Iran could design a submarine in these classes for our friendly nations.