By 717, I would expect the Bulgars to give the Arabs a run for their money. The taking of Constantinople should have in itself been an exhausting undertaking, and the state of the diminished Islamic armies should be considered.
I would have less faith on the Avars, but their capabilities shouldn’t be too hastily dismissed either; OTL, it took several blows from the Franks to finally bring them to their knees almost a century later.
A possible scenario would be the invasion of southern Italy from the Adriatic coast of the Balkans: perhaps the Slavic tribesmen in the northwestern/western Balkans could play a role similar to that of the Berbers in Spain. A relatively small force (perhaps 15,000 men) could defeat any armies in the lower half of the peninsula, at this point divided between the Byzantine dukes and the Lombards of Spoleto and Benevento. In the north, OTOH, we might see a temporary alliance, between the Lombard monarch and the Imperial remnants in Ravenna. Should the Islamic armies be defeated, and the government in Ravenna is recognized as Constantinople’s successor, a love-hate relationship might develop along the lines of OTL’s Byzantium and Bulgaria.
Should the alliance fail, the Pope might take a trip to the other side of the Alps, and seek refuge amongst the Franks. Continuing further north would require more manpower, which might put a hold on the expansion to the East. Should they manage to do so however, expect a much larger battle between the Franks and their vassals (the Alamanni, the Bavarians, etc.) than Tours ever was, as the Muslims would be dangerously close to the center of the Kingdom.