It requiers a PoD in the late 18th early 19th century for the south Slavs not to start calling themselves south Slavs, because they were the southmost of the bunch.
Another thing that needs to be understood is that south, west and east Slavs are basicaly just linguistic denominations created in the 19th century.
A different division based on the mentality and culture could have happened.
Slovenians, Czechs, Moravians, Silesians could be western aka germanic Slavs
Croats, Bosnian and Slovakians could be middle/central aka hungaric Slavs
Serbs, Montenegrins, Macedonians and Bulgarians could be souther aka turkic Slavs
Poles, Belorussians, Ukrainians and Russians could be easter aka slavic Slavs
The greatest obstacle is to prevent south Slavs starting to call themselves south Slavs due to their geography. And the first that used that denomination were Croats and Slovenians.