Each African civilization will have its own play style. They will have some shared units & some unique units to each civilization. The Oyo Kingdom is a cavalry-based Civ with unique hand cavalry (heavy cavalry) & spear throwing cavalry (light cavalry). The Kongolese will have their unique set of units like poisoned arrow bowmen (the poisoned arrow has a small chance to do a "paralyze" effect which will "stun" the attacked unit for few seconds
. Additionally, you can refer to my article on moddb.com for further information
http://www.moddb.com/mods/age-of-empires-3-kings-return/news/age-of-empires-3-kings-return I will be updating my page on moddb.com regularly so you can keep watching if you are interested
.
In AOE3 the civilization you can add is hardcore-limited but there is a work around in the age community to add new Civs will keeping the existing ones. Additionally there seems to be a break-through in modding AOE3 as a patch has been released but the modding community which can enable you directly to add new civilizations.