The Exarchate of Africa was the Byzantine Empire's center of power in the west until it fell to the Islamic Empire in the late 7th century. The Exarchate originally held out against the Islamic Caliphate for a good while, the Caliphate failed to capture the city 686 before doing so in 695, finally in 698 the Byzantine Empire returned and retook the city, a short while later the Calphiate took it back and the city was destroyed for good.
What if in 686 the city of Carthage fell to the Caliphate, would the Muslim's use the great ports of Carthage to launch a massive invasion of Italy? Perhaps instead of Hispania, the Caliphate goes into Italia? I know some who will say they are already in the area, so this changes nothing, but did Tunis of the early 8th century have the ports needed for such a logistics operation, I know Carthage did, unless the Exarchate destroyed them as the city fell.
How would an invasion of Italia play out? Would it be anything like Hispania? Perhaps an Italian reconquista of the peninsula, and a stronger united country in the process? Or will the early Muslims get a foothold in Europe's boot? Would the Byzantine's perhaps focus on "liberating" the people of Italia from the Muslims?