Either arab siege of constantinople succeeds
The caliph sees little reason to expand beyond Thrace into the war-torn, depopulated balkans, and as such the area is left alone.
The slavs continue doing what they'd been doing for centuries.
The Greek language survives on the islands, some remote parts of Morea, and maybe Thessaloniki, if the place doesn't get sacked.
The eastern church (operating from occupied constantinople) may still succeed in christianizing them, as they were allowed to proselytize outside the borders. So Koine may survive in Greece as a liturgical language.
Emperor flees to Italy, his remaining holdings there get ripped to shreds by the Lombards. Oh well, he can always be the King of Sicily... until the Aghlabids come knocking

Maybe then he flees to the adriatic, and instead of doges we'll see (titular, but still!) Emperors of Venice