I would almost say that a lesser known actor, ala Chris Reeve's casting for Superman 78 and more recently, Tom Holland for Spiderman, may be an option.
And I emphasis may be because I don't know if its the best option.
On the positive side, they come to the role with no baggage and a clean slate , and aren't known as playing another well known character.
It becomes easier for the audience to accept them as the new T'Challa than an actor with a higher profile.
They'd have alot to prove, but since they wouldn't be the most high profile actor in the world they're be alot less of the usual " OMG, That guy is playing Black Panther! He's totally wrong for the part!" stuff we see whenever these roles are cast.
The main problem with the " cast an unknown or lesser know actor" route, is that you could run into the issue of whether they have the experience, the talent, the presence , to conceivably portray a T'Challa who's been around a few years to have experienced CW, BP, IW, and EG and has been a King.
An unknown, depending on their acting experience, and age may come off as green.
There is a comfort in casting a known actor the audience already has positive feelings about . While you're always gonna get naysayers whoever is cast, if the actor is already proven , there's alot less concern about putting a billion dollar series on his shoulders.
