[Mastersteef]

....how about Donald Glover? ;P

But seriously, look at this list:
Young, Gifted and Black:12 Rising Movie Stars - The Moviefone Blog

My vote in order of preference:
1. Anthony Mackie
2. Colombus Short
3. Derek Luke

I think they should go with a younger T'challa so that he'd be in the same age range as Evans, Hemsworth, Johannsen...

That being said, if they decide to go older with the character I'd pick Djimon Hounsou in a heartbeat...

Personally I'd love to see the movie have a younger T'challa take his fathers place after his father is killed by Claw (or Man-Ape, Avengers:EMH style) and his hero's journey to earn his place as the new king.