Obviously you couldn't have all of India and Iran have the same culture, it's simply not possible, but it should be possible for their cultures to remain much more united than OTL, right?
 
"More than OTL" can refer to a lot of things. But sure, let's go for it. You'd need to stop the death of the Vedic gods, which is a process that happened since antiquity. But I'll try to go as far as possible

So, let's say in India, at around the time of the Buddha, a prophet is born in the Kashmir Valley, who preaches the greatness of Indra, the sky god and the greatest of the gods, and how he represents the truth, opposed to the great lie of the demon Vrita. Trade routes spread this cult south, to Maghada. When Chandragupta Maurya overthrows the Nanda, he is converted to this cult, and after he conquers India, he makes this philosophy the main religion of India, and the supremacy of India is strong in Indian thought. As time passes, Indra grows further in importance, to the point that later Indian rulers abolish all of the other gods and make Indra the one and only God, opposed to the lie of Vrita.
 
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