Ali Shariati (1933-1977) had been a rival to the Ayatollah Khomeini. Shariati had a basically Communist sort of conception of Islam which he called "
Red Shi'ia", with ideas of class warfare, social justice and so on - with ritual and the like being relatively unimportant. In exile in London he died of a heart attack, though his supporters claimed he was knocked off by the Shah's secret police.
Khomeini (1902-1989) by contrast had an entirely traditional view of Islam, women locked up and so on.
So you could have it that Shariati lives through his heart attack, or the secret police miss him. So that in 1979 when Khomeini returns in triumph, so does Shariati. Various Marxist groups never had wide public support but did play an important part in the violence of the revolution of 78-79. However, in OTL they lacked a figure to unite them. Shariati could have been that man; we know from the Bolshevik Revolution that Communists need not have majority support to take over.
The other groups in OTL were united under Khomeini by the simple expedient of his not mentioning any actual policies except being against the Shah. This would have been much harder for him to do with Shariati around.
When Khomeini returned in OTL, he had said of the current government left behind by the Shah, that of
Bakhtiar - who had wanted fresh elections and plebescites to determine the future constitution - "I shall kick their teeth in. I appoint the government. I appoint the government by support of this nation." A living Shariati would have disputed that, I daresay. So that Shariati joins with Bakhtiar, and Iran has a civil war between parliament and the Marxists, and Khomeini's supporters.
What happens next? What would the Soviets do?