What about a surviving Exarchate timeline - Egypt still gets conquered, but the invasions of Africa fail to succeed, leaving Africa where it is.
Instead, it undertakes the same effects as the Caliphates Indian Ocean trade partners. People in cities convert, and eventually the Exarchate is either converted, or overthrown internally. If we mimic Indonesia and Pakistan, they didn't change their languages to Arabic, even if the language made some headway.
What could be interesting, is if there is a significant attempt to build monasteries to reinforce Christianity in the mountains and beyond, that when the cities start to flip and certain tribes do, they get converted into ready-built mosques.
Interestingly, you can make a legal argument that this would be capable of building a Muslim "Western Roman Empire", which if wanked a bit, could have a world with a Muslim WRE restoration, with a reduced Orthodox (and perhaps converting?) ERE, Egyptian, Persian, and Arabian Muslim Empires, and a mix of Catholic and Pagan Germany. It'd be a very different world.