One of the significant achievements of Black Panther was that it was the first blockbuster film that was written, directed and acted by a predominantly black cast and crew. Its success gave them the platform to direct the industry's attention to the relative lack of opportunities for blacks and other minorities both in front of and behind the camera. Given that, IMO T'Challa will ultimately be recast, because not only is there more story to tell with the character, if you don't recast him that's one less job for a black actor. That said, whoever they cast will have huge shoes to fill. The one guy I could see handling it (Mahershala Ali) has already signed on to do Blade, so unless he & Feige decide to switch things up I don't see an obvious choice out there (I haven't seen Denzel's son in anything, so no opinion on him).