The Russians would've had direct control over Iran. The Communists of Iran were jailed before WWII because they were acting for Russian interests. They were released by Stalin after he invaded neutral Iran.

TUDEH would propably ordered to join the UdSSR and the Russians would have ordered Russians and other ethnicities to settle in Iran so the Iranian population wouldn't be homogeneous anymore.

They would've tried to claim Iran territories as Russian just like they did it with Dagestan, Ossettia,...

By the way: Peter the Great wanted to claim Gilan and Mazandaran. He ordered Armenian families and other Christian groups to immigrate into this territory so the majority was Christian. He failed because the costs were too high.

We never got any compensation for their attempts to ethnocide Iranians.