The five broad regional civilizations that the Saracens should most logically be “split” into would be:
Andalusians - al-Andalus and the Arabic-Islamic civilization of Iberia.
Maghrebis - the western North African sphere: Morocco, Algeria, Tunisia and broadly the Maghreb, with western Libya/Tripolitania fitting here as well.
Egyptians/Misris - Egypt and the Nile-centered civilization, potentially also representing Barqa/Cyrenaica in eastern Libya. Miṣr is the Arabic name for Egypt and Miṣrī means Egyptian.
Mashriqis - the eastern Arab world, principally the Levant and Iraq/Mesopotamia. Al-Mashriq literally means “the East/place of sunrise,” historically contrasting with al-Maghrib, “the West/place of sunset.”
Arabians/Arabs - the Arabian Peninsula itself, broadly representing the Hejaz, Najd, Yemen, Hadramawt, Oman and other Arabian regions.
This creates a fairly natural west-to-east progression:
Andalusians, Maghrebis, Egyptians/Misris, Mashriqis, Arabians
These shouldn’t be understood as five separate “Arab ethnicities,” but rather as broad regional civilizations of the medieval Arab and Arabic-speaking world, each large enough to function as an AoE2 civilization while being far more historically and culturally distinctive than the current catch-all Saracens.
A couple of clarifications regarding the names:
Mashriqis is indigenous Arabic terminology and provides a natural counterpart to Maghrebis. Jordan, Palestine, Syria and Lebanon fit very naturally within a Mashriqi civilization, while Iraq/Mesopotamia could also be included, although its distinctive medieval history could potentially justify separate representation. “Mashriqis” also avoids the problem of using modern national identities: Syrians would be too narrow/ambiguous, while Levantines is essentially European terminology.
As for Maghrebis, I would see them as a broader regional civilization encompassing both the Arab and Amazigh/Berber elements of the medieval Maghreb. The Almoravids, Almohads, Marinids, Hafsids, Zayyanids etc. cannot sensibly be reduced to a purely “Arab” civilization. For that reason, Maghrebis should logically replace the current Berbers, representing the wider Arab-Amazigh civilization of the medieval Maghreb rather than creating an unnecessary overlap between Berbers and Maghrebis.
The two civs I could see beyond this would be Iraqis/Mesopotamians and Yemenis. However, my proposal splits (or perhaps does not split) Saracens into Egyptians, Mashriqis, and Arabians/Arabs. Andalusians brand new civ. Maghrebis is a relabeled Berbers. Net gain is 3 new in-game civs, 4 if Saracens remain untouched.