All of South Asia must be having muslim majority of over 99% and all countries except Afghanistan should be united with the name Hindustan.
This seems unlikely for one reason: very few Moslem countries have become 99% Moslem.
A few have, but most (even some of the oldest) have retained large non-Moslem minorities, or did until massive population exchanges in the 20th century (Pakistan, Turkey). Most Moslem rulers have been content to rule over lots of infidels, just taxing them heavily.
This has been especially true of populous countries, and South Asia (by which I assume is meant the "Indian subcontinent") was certainly populous from an early date.
I think perhaps the PoD has to be a change in the way Islam is structured and preached, in South Asia or maybe more areas, such that it becomes more aggressive and more successful in conversion in Moslem-ruled lands.
It could be something as off-the-wall as a Moslem religious order which specializes in preaching to infidels, and gets really good at it. Or a Moslem regime or movement which preaches an economic or social doctrine that attracts the lower classes of non-Moslem populations (some kind of land reform movement that draws in vast numbers of Hindu peasants and deposes Hindu landowners and nobles).