Medieval Ghana is way worse documented than Mali and Songhai, or many other Subsaharan civilizations really (it boggles my mind how is it infinitely more known than Yoruba people, culture and civilizations, who imho are among top 5 Subsaharan civilizations). It couldn't appear in civ series because it has no discernible leaders at all IIRC (even mythological ones). In Humankind it could appear, but I see no reason why, if Mali and Songhai represent the same cultural area are much better known, and having more "story" to tell than "well there was this sort of desert trade kingdom described by Moroccans as a nice place".
Also, medieval Ghana has the unique problem of having very confusing name, being completely and utterly unrelated to the modern Ghana in both geographical and cultural terms. The reason modern Ghana took this name is literally only because its leaders were inspired by Ghana as a "first West African civilization". IMHO ridiculous reasoning for countless reasons (couldn't you guys reference your own and much much cooler Ashanti civilization?) but let's not go offtopic. Now, this is especially problematic because
modern Ghana is actually one of the most important countries in African postcolonial history, to the point it would be totally legit as a modern African culture in Humankind, so I would rather have this Ghana than quite boring and insignificant medieval one! And they have no alternate names at all, so you either take one or another. Modern Ghana >>>>> medieval Ghana.
So, medieval Ghana would be very far on my dev list of new civilizations.
Adding Ghana to the game is like adding Iberia to the game. By "Iberia" I mean, obviously, the classical era kingdom of Caucasian Iberia, prelude to medieval Georgia (and no this name is not an exonym, just completely random coincidence, to the point medieval Georgians were wondering if Spanish are their faraway cousins because of that

). Sure, you can add very badly documented medieval Ghana or classical Iberia, and generate bonus points of confusion among players, but why do that if you can add medieval Mali, classical Armenia, medieval Georgia or even classical "Iberians but those Spanish", all of whom are much, much better documented and just plain cooler?