About Abbasid possible new Unique Units
Well, the Abbasids do well economically in the late game, but they are disadvantaged in the early and middle game, mainly in Feudal Age (II) and Castle Age (III).
This could be improved by giving them new unique units that aren’t too overpowered for Imperial, or that are situational. Theoretically, the Abbasid Caliphate in its early days had several types of unique units and corps, not widely known like the Teutonic Knights or the Longbowmen, but they did exist, and note “they weren’t camel-riding units”.
Here is a article about Abbasid possible unique units (requires a Free Account), but also in images for educative purpose and to discuss hystorical units:
https://www.jstor.org/stable/48577987
Some of those units are also present in a mini-documental about Abbasid Armies, from “Epic History” channel. The army info began at the 25:00 minute.
New unique building: Amsar
About how to implement them, se me ocurre igual que los bizantinos y Macedonia dynasty, un edificio único para traerlos.
Historically, to transport these multi-ethnic troops, in addition to using logistics such as camels and Persian roads, they also designated “Garrison Settlements” (أمصار) for these troops. In fact, the city of Samarra was created to be a Baghdad 2.0, but with non-Arab foreign troops in cantons.
Amsar (أمصار)
Unique Abbasid building. Allows the recruitment of troops from various parts of the caliphate. Available from the Feudal Age (II). You can unlock a extra unit in castle age (Max:2), each unlock cost gold. All units cost gold. Military wing “Boot camp” now increase his production speed:
1). Al Abna - Heavy infantry, with a bonus against other melee infantry.
2). Daylami Skirmisher - Ranged infantry, javelin-throwing skirmisher, anti-range unit.
3). Nubbian Archers - Nubian archers with +1 attack and +1 range.
4). Diqhans - Early Iranian heavy cavalry against the Abbasids.