What i think can happen:
-The Umayyads gain naval supremacy over the Mediterranean.
-Christian Iconoclasm probably still develops under Muslim rule.
-Once the Umayyad Empire finally collapses and its tribal armies settle into local dynasties, Greece and the Aegean will be the seat of one of them. As with Persia, i can see a cultural syncretism happening with Arabs and Greeks. Perhaps one Greek emir takes the title of imbaratur?
-The Franks remain as the sole "defender of Christianity in Europe", and the Bulgars remain torn about whether they should convert to Christianity and Islam.
-The Russians either remain Pagan for longer, or convert to some (unorthodox) strain of Islam in order to secure trade with the Bosphorus.