Without Khomeini, the devout religious groups wouldn't have a focus. While there certainly was a tendency among certain groups in Iran to put more emphasis on religion in daily life, Iran had the tradition of "quietism", which basically provided for religious authorities to stay away from secular life. So without Khomeini as the "hidden imam", this might as well have stayed that way.
The other powerful group behind the revolution were the People's Mojahedin and the Fedayeen. They were not bound to the Communist Tudeh party, but still had enough Marxist/socialist leanings to make them appear as potential Communists from an American view. If a civil war emerges, there would probably be a similar constellation like in Afghanistan: the Soviets would support the Mojahedin, while the US would start supporting the remaining loyalist SAVAK forces and later switch to religious groups. It's even possible that they would invite foreign Mujaheddin to come to Iran in order to fight the (often secular) Mojahedin.