I'd consider Kenya/Tanzania, and possibly Nigeria, but that's probably about it.
I'd consider Ghana and Kenya, maaaaybe also Tanzania and Senegal but they have weaker arguments for their inclusion, and I wouldn't consider Nigeria at all as a country which could/should be added to civ series as a modern African civilization.
Kenya and Ghana are prime candidates, because they are at the very top of the most succesful Subsaharan countries. You can't argue they are small, as Kenya has population of 53m people and Ghana has 31m. You can't argue they are obscure, as they are fairly well known across the world for being African countries. You can't argue they have no importance, as they are among like top 5 countries most important for the entire postcolonial era of African history, and they are very active members of United Nations. You can't argue they are too poor and miserable to warrant being seriously added to the game, because they aren't that much worse developed than India, Indonesia and Vietnam. You can't argue they have no remarkable leaders, as both of them have leaders incredibly important for global anti colonial movements (as does Tanzania, not sure about Senegal). And finally, in both cases you can't argue they are
entirely covered by preceding precolonial civs, because Akan people are slightly less than half of Ghana and Swahilli culture is only a small part of Kenya. As for Ghana, I'd still prefer Akan civilization, but Ghana itself would be perfectly fine civ for the game; as for Kenya, I genuinely think modern country would be cooler civ than Swahilli.
Tanzania has mostly similar benefits as Kenya, but weaker arguments in all aspects (poorer, less notable, less known etc), while Senegal is here just because of my sympathy for how chill and nice this country's postcolonial history is when compared with most of the continent.
Meanwhile, Nigeria, in my opinion is a bad idea for addition to this series because I consider this country to be in a completely disastrous state nowadays and endemically since its inception, with much worse arguments for any sort of common identity than Kenya/Tanzania/Ghana/Senegal, very clearly divided between several civilizations, alien to each other, in a constant state of ethno - religious bloodhsed for six decades. It also lacks a beloved founding father leader (unlike Kenya, Tanzania and Ghana for example), and civilizations contained within it (Yoruba, Hausa, Kanem, Igbo, Edo/Benin) are entirely capable of functioning on their own in a game like this. Unlike for example my doubts about medieval Swahilli Kilwa civ, which utterly lacks charismatic leader, seems very one dimensional and doesn't have that great documentation at all.