Bringing in Hunter is a must, in my opinion, but It wouldn't want him as the villain. We've had too many familial duels, and Hunter as a character has never really wanted the throne for himself, he only wants to ensure the nation's security. Which sets up a logical introduction for him to return home now that T'Challa is going public.
A bit of a deep pull, but I think Moses Magnum would be a great fit for the next sequel. The fear is that opportunists will swarm in to take advantage of Wakanda, and Magnum Force is a great choice to embody those that would seek to take advantage of open trade to do harm, as well as provide commentary on the military industrial complex. I want to see Namor as much as anyone else, but I feel like that's a huge step for the 2nd film to take. Save Atlantis for capping off the trilogy.