Lupita Nyong'o is who i'd want as Black Panther if i were Marvel.
I would say Lupita or Danai Gurira.
When I watched the original Black Panther I never would have thought of Shuri as a future Black Panther: while she was a good part of the movie she was always to me the Q, the support, not entirely adept at fighting.
On the flip side, Nakia and Okoye where both warriors and strong protectors for Wakanda, even without the suit and the powers of the Black Panther - people who were so strong they could keep up with a literal super hero in fights. It doesn't make sense to me that one of then wouldn't become the protector of Wakanda, but the scientist sister who needs saving could be.
Also I think as pointed out earlier, the fact that Wakanda, the strongest most advanced country in the world, still has a tribal Monarchy system which was decided by a fight, and is now decided by hereditary lines now, seems a bit backwards. I could see possibly for the role of Black Panther decided by combat but why also King/Queen?