The problem is that outside of the coastal towns there really wasn't much going on in Latin Illyria by the time the Slavs got there in the seventh century. The Ostrogoths, Huns, Avars and the simple factor of the region changing hands over and over really did a number on Roman settlement inland. It was a real Britannia situation - or even worse, since the Slavs didn't have the chance to depopulate the settlements themselves like the Anglo-Saxons did.
What you'd need for something like this, I think, is to have a a sufficiently early POD in the WRE that Romans still see an interest in settling elsewhere, starting long before the Slavs even think to get up and move. And even then, the migration can't be as big and drastic as we know it in OTL, otherwise you're going to end up with a Britannia. It'd have to be more of a trickle, I think.