Well, maybe something like that would happen if Persians managed to conquer Constantinople between 616 and 626 (because of death of Heraclius or his flight to Carthage)?
Under certain circumstances that might be the Avars to take Constantinople. The Persians might get Asian part of the Roman Empire.
You could definitely get a situation where the Persians wreck the empire. But I don't think it would turn into a Slavic ERE from there.
The Persians AND the Avars wreck the Roman Empire.
The Avars get the European part of Empire including Constantinople.
The Slavic subjects of the Avars migrate to the Greek-speaking lands and they became much more Slavic-speaking than in OTL.
In a hundred or so years the Avars become Slavic-speaking for the same reasons as the Bulghars in OTL.
So we've got a big empire which has Slavic as official language and the language of majority of the population.
Of course they are Christians and in a hundred years the Constantinople patriarch crowns the Avar Khan as the Roman Emperor.
So we have the German Roman Empire in the West and the Slavic Roman Empire in the East.
Would the Volga become the new center of administration for the Rus? Assuming butterflies still occur, would there be different religions taking root there? I do know that the Orthodox faith will be chosen among the Avar elite..
I guess the Volga would become the new center of administration for the Rus. And consequently the Rus becomes Muslim. One part of the Rus Scandinavian warrior elite becomes Slavic-speaking - in the North-West. The other part would become Turkic-speaking - in the South-East. So we would have two separate 'Russes': 1) Slavic 2) Turkic. But both are Muslim and related as they have Rurikids as their ruling dynasties.