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Find a way for the Zunist religion (TL;DR Sun-worshipping Afghans in the late 800s) to survive the Islamic conquests of Afghanistan, or alternatively have the Zoroastrians survive in Iran long enough that the Zunists manage to reinforce their position, or even find a new place to settle in Central Asia or India.
Bonus Points if you...
Keep Islam out of Iran/Afghanistan/Central Asia, perhaps replace it with Zoroastrianism?
Alternatively, keep Islam in Afghanistan and Co., but have Zunism as the second-largest religion in the region. Perhaps Zunism picks up some Islamic traditions along the way?
Another alternative, have Zunism revived in the 'modern day' as a neo-Pagan movement that has gained a large following amongst Afghan* nationalists as a counter to Islam.