I've said this before that I don't really think there's a way to prevent Iran and Saudi Arabia from getting nuclear weapons in the next 25 years. I really don't. No "treaty" is going to work. No amount of pressure from the U.S. and E.U. is going to change the way these two Mid-East Powers will behave. They have the money, the means keep getting easier and the reason to do it.
It is very likely Iran and Saudi Arabia become the Pakistan/India relationship in South Asia. They just don't like each other. The divide between Shia and Sunni Islam is literally 1400 years old and these are countries that are BASED ON SHARIA LAW. It's one thing to just have differences in your religion. It's another when your entire form of government is based on that religion.
Both sides think it's their God given right to influence the Arab world towards their version of Islam.
This is kind of the world we live in. North Africa is going to be radical Islam and chaos. Iran and Saudi Arabia are going to fight over Shia/Sunni Islamic fundamentalism. Pakistan and India are going to fight over Kashmir. Afghanistan and the mountainous central Asian Muslims are tribal Taliban factions.
The rest of the middle east is fractured puppet states and dictators (some more benevolent than others).
And smack dab in the middle of it all is Israel just doing their thing to survive basically just walling themselves off from the hoards.
Honestly, it's been like this for 50 years (since oil was discovered and gave them global power/influence). Nothing is going to change.