In Africa, you could add Wolofs, Songhais, Mossi, Yoruba, Edo, Igbo, Hausa, Kanem, Bornu, Alodia, Makuria Kongolese, Zimbabweans, Swahili, Ajuraan, Akan, Luba, Lunda, Fula…
Malians and Ethiopians are some kind of ridiculous umbrella civilization supposedly representing West Africa resp. East Africa while sharing one architecture set.
I think there’s some areas in Asia which could definitely benefit from more civs like Central Asia, the Sinosphere, the Indosphere as well as insular Asia.
The Inka campaign is a sign there’s some off-priorities. In South America alone, you could add Chimu, Muisca, Huari, Tiwahuanako, Moche, Mapuche, Tupi etc.
Central America is also missing a lot of civs ranging from Tarascans, Zapotecs, Mixtecs, Toltecs, Tlaxcaltecs etc.
North America, my personal favourite possible future DLC zone could have the Mississippians, Hitsatsonim, Hohokam , Tlingit etc
Oceania could have T’ui Tonga, Maori, Samoans etc.
The population and cultural diversity in every one of those areas is huge and they’d be perfect occassions to add new architecture sets for each new culture zone. Campaigns would be based on oral traditions of their respective regions.