I think you need a POD of well before the rise of Islam. How about this *very* breif timeline.
Western Roman Empire does a bit better and holds on 30 more years before collapsing circa 500 AD. They however lost their Balkan Provinces, so the Eastern Romans conquer them *Aquinium to Salona area* and then, for whatever reason, be it a de jure Emperor in Rome or whatever, do not push for Italy.
This means the Eastern Roman Empire controls the primary route into not only Western Europe used by Barbarians of the past, has the wealth of the Balkans, and the Slavs are now faced with a very powerful and not bloodied overextended rampart all the way along the Danube to Germany, where the tribes of OTL that might not have moved into Roman Territory *Lombardi, Burgundi, Alemani, not sure, collapse of Rome in the West is unclear* would stand as a bullwark.
This means that the Slavs will be forced to go south, maybe through the Caucuses. Combine this with a stimulu to do so *Magyrs, Alans, ect...* and a weakened Persian Empire due to Roman focus on them rather than the west, and you have a recipie allowing slavs to setup states in Northern Iran, the caucuses, and eastern anatolia and maybe even Northern Iraq.