How about some more civs that predated cultures found in AoE2 & 3?
- Aksum - (as great as Rome, China, Persia) (pre-Ethiopia)
- Bagans (pre-Mayanmar)
- Harappans (Indus Valley)
- Olmecs (pre-Mayan)
- Toltec (pre-Aztec)
- Caral-Supe (i.e. Chavín, Paracas, Nazca, Huari, Moche) (pre-Inca)
- Mississippia (i.e. great copper civilization) (pre-Suix, Choctaw, Dakota, Shawnee, Kickapoo, etc)
- A Northern Europe Culture (i.e. Picts, Pitted Ware culture, Battle Axe Culture, Hallstatt culture)
- A Polynesian culture (i.e. Aotearoa, Hawaiʻi, Marquesas, Samoa, Tahiti, and Tonga) (pre-Pacific Islands)
I think most if not all of these offers very unique units, gameplay style, tech, and design for the game. Helping diversify gameplay in a way that things like technology during that time period could not.
Israel, Nubians, Numidians, Mongols (Xiongnu), Gauls, Iberians, Brits(Picts or we they were called), Celts, Mitanni, Arameans.
A Celtic civilization is a must. At their peak, Celtic civilizations existed throughout Europe, including Ireland, England, Spain, France, Germany, Italy, Switzerland, Hungary, Romania, Ukraine, etc. They even had a civilization in Turkey. A group of Gauls actually sacked Rome early in its history, and the wars between Rome and various Celtic tribes were numerous.