[userulluipeste]

Dacia (Felix) was the Roman Empire's richest region in gold, and this was a very advertised fact. Emperor Trajan's triumphal entry in Rome had in full display the spoils of war, and it was the richest in gold and silver in the entire Roman Empire's history. It's expected that, after "pax romana" went into effect, there to occur a gold rush like the one that happened in USA, drawing massive immigration to the region from all over the empire. Here's one of the gold mining settlements, founded at that time: https://en.wikipedia.org/wiki/Ro%C8%99ia_Montan%C4%83

Considering that Dacia Felix was the last incorporated roman province, so the pool of potential immigrants was literally from the Roman Empire at one of its largest territorial extension and thus -- of the highest ethnic diversity. The fact that the region got mixed up genetically and now resembles that of the rest of the Balkan peninsula could be explained by later events. There was massive Slavic migration to the region, then there was significant depopulation of the plains surrounding Carpathian Mountains when Mongols invaded (and periodically raided the region for loot and slaves), and then there was also a massive wave of christian refugees fleeing from the Ottoman Empire expansion to the (still christian) areas north of Danube.