There is one Weakness as well and that is Persian Nationalism. I heard that Ahmadinejad was well respected among Iranian not because of his Aggressive Foreign Policies but rather Pride in Persian Heritage of Iran and that was not a favorite thing for Iranian Shia Theocracy so he was considered Moderate Force in Iran while Current Iran PM is well in line with Iranian Theocracy. So Promoting Persian Nationalism is a way but for Arabs this would be worse Idea because they are also promoting Arab Peninsula Based Bedouin Nationalism which is direct in conflict with Persian one since Khilafat when Sassanian were taken over.
ISIS have also said against Saudi Regime as well in fact about Makkah and Medina as well so they are against Saudis in the region but others are just PROXIES rather then a ORGANIZED ARMIES while what Iranians have built in Arab World is ORGANIZED ARMED FORCE like Structure with Examples like Mehdi Army, Hizbullah, Houthis etc. They are not easy to control so Saudis have to make a huge sacrifice in their Position towards ISIS if they want to Achieve anything. Even that wont answer the question of Persian Gulf because all Saudi Oil Supply depends on it and Iran does have Capability to Choak that.
Nationalism is not a bad thing and not the reason of conflicts. Arabs, Persian, Turks will remain, their nationalism will remain. It is not logical to connect nationalism to conflicts. It is intolerance, injustice, rejection and occupation that is the reason of conflicts.
The Saudi is not against IRAN due to arab or Persian nationalism or shiism. It was all normal before the Islamic Revolution.
Saudi+Allies get shocked due to success of Islamic revolution in IRAN as Shiites in Asian part of middle east are not a minority and they all fear revolts in their own countries too. they even imported millions of immigrants from all over the world to GCC and West started huge investments in desert nations which is now called GCC.
They started creating proxies and even invaded new born Islamic IRAN. they tried to bring fake revolutions in arab countries but that all backfired.
It is NOW again clear that despite wars, sanctions, threats, killings, the Shiites cannot be marginalized as they are not a minority but are significant communities.
So Saudi+nato allies has to decide what to do with their proxies isil, Qaida, Taliban, let, jud, ji, lej and dozens more they created to counter Shiites.
If they want to use their last card turkey+pak+nato, they should try that too. That will also be a failure.
Those who support Saudi are Either intolerant Or they do not understand the nature of conflict and blame nationalism.