I mean, returning "again"?
They've been in exactly one game. We could let them return once before taking issue with them returning again XD
(And I mean, really they're kind of the obvious choice for that region).
The way I see it, Kongo should be the go to Central African civ, occasionally replaced by another (but let them have two turns first). In West Africa, meanwhile, we really need two civ slots, with Mali getting promoted to full-time series regular and another civ (Songhai, Ashante, Hausa, what have you) rotating for the second slot.
In Southern Africa, there's dozens of interesting picks (everythin to do with Zimbabwe/Mutapa, the southern parts of Swahili land...) but we'll never see any of them because my least favorite Nuclear Gandhi-grade stupid civ meme has a stranglehold on representing the region and has already gotten in about five more games than they ever warranted.