If there are no sanctions, or if there are sanctions but they have no significant effect positive or negative, then here are the possiblities in the next level:
(1) Nothing happens and Iran gets nuclear weapons. This is the most likely outcome because the U.S. is afraid to act and no other country has the power or desire to stop Iran. 55% probablility.
(2) Israel attacks Iran. 10% probability.
(3) Secular Iranians overthrow the religious government, and then voluntary end the WMD program in exchange for U.S. aid. It hasn't happened since 1980, why is it suddenly going to happen just in the nick of time to save us? Highly unlikely. 2% probability.
(4) U.S. led attack on Iran. 33% probability.