They don't have one at all...and the movie has disaster all over it. From the beginning, they should have recast the role. It gets them out of any bind, and leaves every opportunity open, and they don't have to shift the movie to one by committee. Once the decision was made to not recast, whatever future plans Marvel had for T'Challa were thrown out.
I've said it numerous times through the past 3 years. They really didn't have much of a plan for anything after Endgame. They had Spidey 2 already in the can, and the only sure things at that point was BP2 & Captain Marvel 2.
Thor 4 wasn't a thing until they saw the reaction to their chemistry in Endgame, Hemsworth said he would continue to play Thor, and Guardians 3 was shelved. Dr Strange 2 didn't have a script or even a greenlight. Even Shang Chi was fasttracked after they saw the success of BP.