This is roughly what the population density of the Balkans looked like right before the Slavic invasions. The region was severely depopulated and only real area of interior settlement is in modern Albania. This area is known as the Komani culture.

The Komani culture is asserted to be Proto-Albanian by some Albanian scholars, but it appears to have been a romanized group. It’s likely that the people living there went extinct and this may be the source of the Western Romance language mentioned here:

Whatever the case, the Proto-Albanians must have been a small group. DNA evidence suggests that, and Albanians experienced a rapid expansion in the late first millennium. Albanians would have lived in the mountainous areas of Roman Dardania.

My personal theory is that Albanians who moved into modern Albania eventually merged with the Komani people and since the Latin influence faded, they began speaking Albanian, hence why there is so much Western Romance lexicon in modern Albanian.

It could also be that the Komani people were fully Romanized Albanians (so to speak) and they lived side by side. There are certain Paleo-Balkan pagan motifs in their gravesites. They would have been part of a network stretching across the Vis Egnatia.

Since Proto-Albanians were clearly a pastoral people (as told by the rich vocabulary in animal husbandry), it may very well have been that there was a sociolinguistic divide in the region. This was very common in Roman provinces, eg. in France.

So the people living in the Komani area were Latin speakers whereas the people living in the surrounding environment were speaking Proto-Albanian and they interacted frequently. The fate of the Komani people is uncertain, but they may have been subsumed into what became Albanian.

This would be unusual, but not impossible. It was clear that by the first mention of the Albanians in the 10th century that they were dominant in this region. It may well also be that the Komani people had left.

Regardless, since they were part of a network stretching from Italy to Macedonia, they most likely spoke a Western Romance and not an Eastern Romance language. So they would be the unidentified Western Romance language mentioned above. The geography fits perfectly too.

The Balkans were very heavily depopulated by the seventh century. This explains why Byzantine Emperors invited the Slavs to settle along the Danube. There was no real conquest there. The raiding took place much further south in Greece and Thrace. They were joint Avar-Slavs raids.

So aside from some populations in the South, the Slavs did not mix much with Paleo-Balkan peoples as there weren’t many to mix with.

The only Paleo-Balkan people to exist besides the Greeks were the Albanians, and they most likely resided alongside the Komani in the 7th century but it’s likely they extended as far as Nish and Shtip before then.

They were most likely a remnant of the Dardanians who were forced into modern Albania as a result of Gothic and Hunnic invasions.

We know for example Belisarius and Justinian were Romanized Illyrians and both came from areas to the east of Albania. So the Dardanian-Roman presence was much stronger there before the 7th century.

Albanian is a remnant of a non-Romanized but heavily Latin-influenced language. Like I said, it was likely a sociolect of sorts spoken by lower-class people’s living in the countryside and the mountains, while their Romanized relatives lived in the cities.