There’s two PODs here. The Roman POD would kick off the camel caravan trade 500 years earlier. IOTL the Ghana Empire was built on the salt and gold trade around 700 AD, ITTL we could see a similar empire around 200 AD. The Romans would trade with them and introduce writing and Christianity. Literacy may rise among the elite a thousand years before the Muslims brought it with them. The Roman North African provinces would be wealthier and more important from the gold trade. By the Middle Ages, West Africa would be more cosmopolitan. It wouldn’t just be a mysterious place where a handful of famous travelers like Ibn Battuta had seen.
A Carthaginian POD would be trickier as they would start a sea trade which didn’t exist until the Middle Ages. Some say Hanno the Navigator visited West Africa around 500 BC. If trade colonies were estabablished back then, as was the Phoenician pattern, their language and maritime package would be adopted by the natives. That’s quite a boost from dugout canoes. It’s possible the Carthaginians, rich with more gold do better in the Punic Wars.
It’s hard to see how it would be possible that West African sailors would not discover the New World before anyone else. Brazil being so close and the currents perfectly favorable. Were that to happen West Africa would greatly benefit from corn, cassava and sweet potato as these are more productive than the analogous native sorghum and yam.