I don't think Denzel can do an African accent. He didn't do one for the Gladiator sequel because he thought it would be disrespectfully bad. So whoever he plays will probably be an American.
That's an interesting idea about Killmonger, though. Maybe in an alternate universe where he managed to keep the throne, Killmonger conquered the world and realized the problem of violent revolution. So now he's a mentor to Touissant, but leading him down a questionably violent path.