Coupla things:
a. There aren't Christian and Arab faces

the game divides into "testfaces" and "darks." Still, until ck2 and probably beyond, there's only the two classes. From some of the talk on the boards, it looks more likely that if a third class is added, it'll be dark Asian features to represent the Mongols separately.
1.06 is supposed to remove Nubia even though the current setup is correct - since the 970s, Nubia occupied the southern 3 provinces in Upper Egypt.
b. This region was inhabited by the "Red Nuba" (/Noba) and not the "Black Nuba" - they'd been intermarrying with the Egyptians for millennia and the Arabs for centuries. The Nubian aristocracy was fond of Byzantine "princesses." So it's mostly correct on the current setup anyway.
c. That said, there were also plenty of "Black Nuba" in the region, especially - one would imagine - in the royal court and royal line (I've only seen a few murals from the cathedral at Pakhoras / Faras online but they're all of Black Nuban people with white angels or religious figures).
d. It'd be nice to have a separate, rare, tier of darker than "dark" faces for Berberia (from the trans-Saharan trade route), Nubia, and Egypt (they had 50k+ "Nubian" slaves, mostly in the military but sometimes as Viziers).
e. If anyone wants to help or point me in the right direction for how to do it myself, I think it'd be great to add one black female and one black male portrait whose DNA would only show in Nubia at startup but could randomly appear elsewhere afterwards.