Apparently, for the De Administratio Imperii, it was the proto-Croats that settled Belgrad first (or at least a slavic people more tied to Avars than proto-Serbs).
A weaker Avar Empire during the VIII century could lead the slavic people around Belgrade to be part of the Serbian ethnogenesis.
Now, you'll need something to have this one being focused on North and not on Adriatic coast : maybe a weaker Byzantine presence there would do it (letting the way for Lombards, Carolingians, Byzantine rebels, or even Arabs to raid the coast).
Marko have probably more support than me in this question though.