So after 1718 till 1740 Serbia was ruled by Austria. After the loss of a war against the Ottomans they lost it but at that time i dont think that it would be that hard to butterfly away that loss. What would be the effects if Austria retains Serbia?
Secure a Habsburg male heir, so Charles VI don't allies with Russia to make the Tsar recognize the Pragmatic Sanction and lead to the disastrous war in 1740.
I dont think that the war is necesserily disastrous - they just need to avoid loosing as badly as OTL. They dont need a male heir for that.
Religion i dont see as a problem as the Empire already had a significant orthodox populace and there was no problem because of that - not even later.
Serbia is for sure handled seperatly from Hungary. The Military Frontier is organised in Serbia instead of Southern Hungary. Without this loss no Serbians fleeing the Ottoman reconquest means that southern Hungary will be more german and hungarian. Serbia is likely properly integrated and is much closer to the west culturally as well. In Croatia there still will be part of the Military Frontier but much larger part of it will be under civilian administration. Less serbs here too. However Croatia at this time ment Croatia and Slavonia. The difference was that the Slavonian regions sent representative to the hungarian parlaiment while Croatia didnt.
This happens in th 18th century. But i have a hard time guessing how thing will continue in the 19th centry.