Another idea: If Iran successfully prevents the Muslims from taking over, it could have a new religion arise within its borders from the decaying Zoroastrian religion of the time, like Mazdakism and Manichaeism did, and eventually take over Iran and/or Central Asia.
Or just have Manichaeism take over Central Asia. It wouldn't be founded after Islam, but it would only be considered a major religion in size after Islam.
Or just have Manichaeism take over Central Asia. It wouldn't be founded after Islam, but it would only be considered a major religion in size after Islam.