Global Caliphate in three easy POD's!
1. The Caliphate takes Constantinople, and the Byzantines never recover, fleeing to Syracuse. The Bulgars, Rus', etc. are exposed to Islam, rather than Christianity. Khazaria and Georgia are Ehtiopia-like isolates, maybe.
2. Andalusian victory at alt-*Tours* and the follow-up campaign brings Gaul, Italy, and eventually Britain under Islamic rule. The Saxons and Northmen are exposed to Islam, rather than Christianity. Ireland is an Ethiopia-like isolate, maybe.
3. There is still something like the *Mongol Empire*, but the leading tribes are Muslim. There is a lasting Muslim plurality (or sizable minority at least) in North China. It's not implausible that this empire either conquers for a time or otherwise influences Japan and/or Vietnam, but it's not necessary.
In this world it is Islamic powers (Western, Eastern, or both) who lead the alt-*Age of Imperialism*, meaning that the Americas, Australia, North Asia, and sub-Saharan Africa are at least 90% Islamic.
Et voila.