I'm sure there are some solid fuel/chemical related inputs that only the Saudis can supply due to them being a major oil exporter too. So, with investment and R&D, they too would need to establish industrial units leveraging their strengths (of which there are many). Hence, in addition to parallel production lines, there is an opportunity for actual, industrial-layer integration between KSA and Pak.
China, India, etc., always make it a win-lose issue; we can make it win-win for the publics and nations involved.
Saudis are a leading exporter of DAP; Diammonium Phosphate, which among its many uses can be used as a thermal insulator for rocket motors.
For the Saudis, bringing in Pakistan as the contractor for strategic tunnel projects would be best placed for ensuring secrecy. Pakistan for its part would not want to harm this most valuable of partnerships and would do its utmost that it vetted anyone on the projects as well as prevented any leaks, to the best of its ability.
I don’t think it’s fair to compare Saudi and China. Frankly, all partners of Pakistan have interests that converge. We have seen in recent years where Saudi frustration with Pakistan has lead (in part) to change in Pakistan.
If there is to be deep cooperation, we have to look at where each nation could be of strategic benefit to the other in the economic domain, and frankly on a competitive basis.
For Saudi food security, especially after the danger of their water plants being threatened by Iran, it doesn’t make sense, IMHO, for the Saudis to deplete their water reserves and keep building water plants when they could be better off partnering with Pakistani farmers and helping to build out Pakistan’s water management infrastructure (as well as leaning on India to stick to the indus water treaty).
For the money they pump into the water plants, if they poured that into the “Pakistan flood control system” (2010/2011 proposal), as well as spearhead agricultural modernization, we would see arable land increase from 45 million acres to 70 million acres and crop yields increase 100-300% depending on the crop. The food could be exported to Saudi and other nations, and yield all involved high returns. Pakistan would then be a growing consumer of Saudi oil and gas and fertilizers.
This is the lowest hanging fruit for cooperation, alongside the pipeline from Saudi to Oman to Pakistan and on to India, IMHO.