first local people become seafarers
this includes what, illyrians, dacians maybe some celts doesn't really matter, probably illyrians colonize the islands, dacians were barbaric and celts were scared of water
then they are conquered by romans, perhaps northern part becomes vassal kingdom, akin to pontus, same might happen with the eastern part; border of the empire follows the south-western coastline and danube, but islands are eventually incorporated as allies or conquered and romanized
roman identity survives imperial collapse, but barbarians intermingle with locals and conquer the coasts
steppe people (huns, avars, bulgarians and magyars) go to plunder south, rather than west and eastern empire losses it's european possesions, with the exception of constantinople ad, possibly, peloponnese; maybe its even totally destroyed in the byzantine-sassanid wars
meanwhile, pannonian islands and coastal cities turn into small principalities and city states, remaining under lose control of rulers of italy, and, later, frankish empire, eventually becoming part of the hre, and forming united nation state(s?) in XIX or XX century
basically discount italy