How could The Balkans look without any strong Byzantine presence?
It'll be mostly Slavic, that's a given.
What of the Avars?
I don't know. I would say the Magyars would conquer them as in OTL but they might stick around in Pannonia (good horse-breeding country) and remain a distinct polity though it's likely they'll be Slavicized or Germanized depending on where they settle.
Couldn't they settle in Bulgaria to take advantage of the power vacuum?