View attachment 326915
POD: Hilderic survives and the Vandals become a puppet Kingdom rather than being outright annexed.
Current Year: 1839
Justinian's long reign focuses more on the fight in the East with the Sassanids rather than the west, being blocked by relatively friendly Kingdoms. As result most of the Middle East falls under Byzantine domination rather than the former Western Roman Empire. However, the wars are far far more costly as result.
A rump Sassanid State left in the main Persian Plateau, and is forced to accept Christianity as another offical religion.
By the time Islam and Muhammed come around, Byzantium is facing lots of problems regarding the indigestable population in the Middle East. As result the Byzantines are routed and overrun. The less prideful Persians accept the new state of affairs (Muslims never went after them because they never controlled Mesopotamia or the Levant.) A few Byzantine holdouts exist in the Aegean and in the Pontic region.
Now having Constantinople as it's capital, the Caliphate claims the Legacy as rome as a whole, eventually conquering Italy. Germanic tribes and settled steppe peoples begin to convert to Islam rather than Christianity.
Meanwhile succession wars begin to wear down on the Caliphate, who slowly lose their middle eastern Homelands from homegrown Egyptian revolts and a new Aggressive Persia under a Nestorian dynasty, as well as a Pontic Byzantine successor state.
Meanwhile, the fate of European Islam is sealed with the conversion of the Franks. The fate of Christianity is upheld by the Arian Visigoths, who've forced the Franks to mostly expand into Northern Germany rather than the rest of Gaul. This is coupled with the extremely stubborn Alpine nation of Bavaria who refuses to convert.
Over time Frankish attacks and internal instability wear the Visigoths down far enough to their collapse, and the eventual replacement with an Islamic counterpart. Arian Christianity still survives today in Southern Spain however under the Kingdom of Algarve.
Throughout this time the Persians and their followers begin to expand through India and the Steppe, converting much of Northern India to Christianity. However, rumors persist of a small "Sultanate of Aceh" that survives as the last Muslim nation in the East.
The viking age lasts much longer than OTL as the Germanic pagans of Scandinavia are less willing to give up their faith due to love of beer, previous interactions with Christian refugees of which they've managed to incorporate some stronger aspects of their faith into, and much of the Islamic diet being incompatible with their usual diet. Nonetheless, they are eventually converted and almost completely eradicted. This comes with the help of Uralic peoples, notably the Finns (Finnic group maintained it's larger nature in TTL) Islamified vikings eventually make it to America followed by the now islamified Goths who land in Brazil.
Around the same time the Celtic Sultanate is formed in Ireland, and begins to subjegate the Anglo-Norse states which have been plagued by years of war.
Time goes on, history happens. More will be revealed in the full map, but feel free to ask questions now.