If you want to prevent something like the rise of the Islamic republic in Iran, I think stopping the secular reforms of Reza Shah and his son is needed. They both wanted to emulate Turkey, which under Ataturk had enacted a number of sweeping secular laws which pushed Islam out of the public sphere to a large degree. However, both father and son ignored that there were a number of differences between Turkey and Iran that made their reforms much more likely to experience internal opposition that eventually manifested itself in the revolution.
The Iranian "Reformists" lacked both Turkey's able body of civil servants, and was faced with a much more entrenched religious hierarchy than Turkey did. The largely cosmetic reforms succeeded in riling up the religious leaders, and did little to improve the position of the average Iranian. If the reforms are much less severe, and the benefits of Iran's growing economy are more evenly shared out, maybe there would be less popular anger against the Shah for Khomeini to take advantage of.