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.
We also have the hanging Namor storyline. Is there someone Namor could be fighting that requires teaming up with Wakanda? Or someone Wakanda could be fighting that requires teaming up with Namor?
Right now the two nations are at peace, but only barely.
If someone can cause problems for them both it's a good reason to have them form a more permanent union. Depending how Captain America: Brave New World goes, since it seems to have a focus on the Celestial in the ocean, that might give them something in common to look at. Namor focused on the ocean, and Wakanda focused on Vibranium vs Adamantium.
Adding to that I think a future T'Challa/Killmonger being seen as a leader, but providing the wrong message could be interesting. Maybe they've already been through this storyline, but made the wrong choice. Not to say they made the story choice, just one that doesn't work in this universe. Assuming the wrong thing. Or going down a track that only leads to a poor victory.