When they filmed the passing of T'Challa, the emotions weren't just acting. All of them were grieving the loss of Chadwick Boseman, and the emotions on-screen reflected their actual emotions.
Cast members such as Letitia Wright have refused to discuss the future of Black Panther, considering Chadwick Boseman will not be a part of those at all.
Considering the key role he played in the franchise, it's difficult to replace him without facing backlash from the audience. The only way is to have someone step-up as T'Challa and make it a natural transition rather than replacing him with a new actor!