Wouldn't stopping Slavs at Danube be enough? After all Balkans were full of Latin population before Danube limes was broken.
Would it not be plausible to the Slavic tribes move into the Caucus or further into Germany instead of the Balkans?
The most latin-settled aeras were either the coast and the Danube basin, while Illyrians as a distinct group are still mentioned at the same time Slavic peoples came in (while probably more on the highlands).
If not Romance, what if East Germanic tribes, Goths, Burgundians, Vandals, had split and settled there (in addition to conquering Rome, Iberia, etc)? Would the Vandals moving to the Balkans instead of Africa be that big a change? Could those areas maintain Germanic-speakers to this day?
It all comes down to the Byzantines holding the northern frontier and preventing the Slavs from invading. The critical time period is the 6th to 8th centuries when the Slav migrations into the Balkans occurred. That either requires the Byzantines holding them back, or that the Eastern Germanic tribes on the Byzantine frontier survive and prevent the Slavs from overruning them.
The problem with the Byzantines holding the frontier is that they are very stretched during this time period. There is the ongoing ulcer of the Gothic Wars. The plague of Justinian. The Persian conquests. The Islamic invasions. These will constantly divert resources that could have been spent beating back the Slavs.
Therefore, I think that the Eastern Germans need to do it. So the Lombards can't invade Italy, and the Gepids need to survive. They'd be the buffer against the Avars and Slavs moving in.
A critical thing is to not have Italy be so inviting a target. Either Justinian needs to get along with the Ostrogoths, or he has to better support Belisarius in his initial invasion of Italy so that overwhelming force ends the war quickly, and the Ostrogoths accept Byzantine rule. The Lombards and Gepids stay in place and somehow defeat the Avars/Slavs; which probably requires a series of strong leaders.
I don't think you'd ever get Bulgaria to be majority Latin speaking. That will remain Greek; I don't think it was ever predominantly Latin speaking among regular people. But you could preserve a Latin presence in the lands of Yugoslavia/Adriatic coast that preserves an indeigenous Romance language.