There were actually several flavors of Christianity in Africa at the time of the Arab Conquest.You had the Church of Carthage,the Donatists, and the Vandals had Arianism,Nicene,and Chalcedonian Christianity.The Egyptians were Coptic as were the Ethiopians at the time. So,I foresee difficulty in conquering Egypt or African Horn,and there might be some hard feelings regarding the Catholic/Orthodox schism. But,there is the strong likelyhood of unifying North Africa, maybe bringing Sicily and Spain in the fold and setting up trade colonies in West Africa. Maybe they'll set up colonies in the Canary Islands as well. I don't know if the Vandals would survive or not,probably not as dominant,maybe as a relic ethnicity on the fringes.