I just don't see this as possible because (1) for Iran to incorporate the Iranian peoples of the USSR (which would include the Tajiks as well as the Ossetians--it would presumably not include the Azeris, who are Turkic) would be impossible without another world war, (2) with regard to Afghnistan its existence as an indpendent and neutral country was also important to the USSR (and supported by the UK and US as well), (3) trying to acquire Iraqi Kurdistan would bring about a conflict with the British (and would also be disapproved by both the US and the Soviets), and (4) trying to acquire Turkish Kurdistan would be especially opposed by the US (which took a special interest in Turkey under the Tryman Doctrine)
In any event, Iran is far too busy with its own problems to have dreams of such expansion. It would be like Italy deciding to conquer and unify all Romance-speaking peoples...
I suppose that theoretically if the Soviets gained control of all of western Asia in a third world war, they would control all Iranian peoples, but there is no reason to think they would group them all together in a single "Iranain" SSR any more than they did all Turkic ones in a "Turkic" SSR.