Suppose there is no Islam. Arab tribes convert to Christianity centuries before that or Gallus conquers them and brings them under Augustus or something like that.
What are the effects and different scenarios that could be unleashed in North India,Afghanistan,Central Asia? More Iranian and Zoroastrian settlement and influence? Different type of Open Hinduism or Buddhism takes root in these regions and others from the North and the West convert? Different ethnic makeup in these regions? You can think of many more scenarios. You are at full freedom to post the relevant timeline of your own.