Well in that case you would probably have a bigger Serbia, i dont know how it would work during the 15th century though. If they were all catholic, maybe they would instead become Hungarian vassals in order to keep away the Ottomans and later you wouldnt have the concept of Yugoslav among them. Now if they were all Orthodox instead, there still would be split, since neither Hungarians nor Turks would be welcome. But in the long run, you would see something more like Greater Serbia rather than Yugoslavia, as the vast majority would consider herself Serb, and the national identity wouldnt be about religion.