Not sure how feasible the following is though the challenge is to establish a Central African analogue of Ethiopia, capable of covering territory from most of Cameroon and Central African Republic in the north to most of Angola and Zambia in the south.
AFAIK the only OTL kingdoms in the area were the Kongo, Lunda and Luba and am unsure of their ability to conquer the rest of Central Africa, let alone become suitable analogue of Ethiopia.