Vladimir of Kiev chooses Islam, leading to the Islamization of Russia? Alternatively, the Golden Horde places the Russian princes under direct rule. The Horde remains a Muslim state, but gradually become Slavicized.
(I think Russia/Ukraine/Belarus is most likely. The West Slavs are too close to the HRE. The Balkan Slavs are an outside shot, maybe as part of an alliance against the Byzantines, but there were already plenty of Christians in the area and Muslims had no problems OTL making such alliances with non-Muslim states; enemy of my enemy and all that.)