May I make a suggestion @wowgetoffyourcellphone?
I propose to add the Kingdom of Pontus to this list. They were able to field huge armies the size of the armies of major Hellenistic Kingdoms, and they could challenge the Romans multiple times. Their unit roster may represent the diverse ethnic background of the region to include:
Chalkaspides (pikemen from Greek settlers)
Bosphoran archers (later kings of the Greco-Scythian Bosphoran Kingdom were from Pontic royal line and were allies)
Cappadocian cavalry (Kingdom of Pontus included Cappadocia)
Corduene slingers (Anatolian people skilled with slings, may be ancestors of Kurds)
Colchian javelineers (Ancestors of Georgians)
Armenian cataphracts (Kingdom of Armenia was an ally, this would be a chance to include more cataphract units which you wanted to differentiate from regular spear cavalry)
Scythed chariots (known to be used by Pontics, another chance to include a unit which you wanted to differentiate from other units)
Bastarnae mercenaries (known to be hired by Pontics)
Getae mercenaries (known to be hired by Pontics)
Possible heroes:
Mithridates I: founder of the kingdom, built many cities
Mithridates VI: 3 times challenged Rome
Pharnaces II: also challenged Rome
I think all of them can be covered by existing artwork with the addition of some shields/emblems. As for the buildings, a mix of Seleucid, Hellenic, and Persian art may be used.