Honestly, I think it'd be a cool PoD pair to have one lead to a stronger Nubia, but then combined with an alliance with the Romans that effectively leaves Nubia in control of Egypt as a result of a joint effort to turn back the Caliphate.
Perhaps an observant Nubian ruler starts reforming when they notice the rapid spread of Islam in Arabia, and expect them to invade the Romans/Persians, meaning Nubia wouldn't be safe.
I mean, hell, that isn't much of a divergence from OTL - if the Nubians and Romans can manage to organise an invasion (sneaky messages being snuck sneakily) then Makuria could well lead to the containment of Islam to Arabia, and become crazy rich from selling grain and goods to the Romans, and being their intermediary for the Indian Ocean trade.
This at least would limit any expansion of Islam into Nubia proper during a crisis - perhaps instead being taken advantage of by the Romans.
I'd find a Makuria-Rising scenario pretty cool. Unexpected beneficiary of the Caliphate.