You need to have bigger, more coherent Sclavenic political entities from the very beginning, as a minimum point: these bigger entities will be more open to the influence of the Church. You need to avoid Leo III transferring control over the Diocese of Illyricum from Rome to Constantinople. You need, if possible, to get rid entirely of a Greek speaking Byzantine state which is always going to be the main root of influence for this part of Europe in the seventh, eighth and ninth centuries.
I see two potential PODs.
Firstly, a more successful and "sticking" Justinianic conquest of the West: let's say, for arguments sake, a quicker conquest of Gothic Italy that means the Adriatic becomes a secure lake of Latinity that's locked into the empire. A wealthy Italy is in a position to contribute to the defence of Illyria when the Sclavenes and Avars come knocking, and it also means the Latin speaking regions of the empire retain political prominence for longer.
Secondly, a POD in which Constantinople falls to the Arabs, who then become relatively bogged down in fighting, and don't advance particularly deeply into the Balkans: perhaps something similar to the OTL extent of Persian rule over Thrace and Macedon in the fifth century BC. The only major "civilised" influences over the western Balkans will then be Latin: either the Papacy, or perhaps a newly formed WRE based in Ravenna.