Co-writer Christopher Markus said the key, when adding new characters, is asking, “Do they have an organic place in the story? Panther has a great place in the story, and Wakanda has a great place in the story. So that even if he wasn't the seed of a franchise, he'd be a good character with a justified place in the movie.” Markus noted, “There were times early on where we were getting character overload, where [we thought] ‘Maybe that just outta be Joe Blow from wherever, not Black Panther,’ but it's too good. It's just too good.”
Markus said that Black Panther, AKA Prince T’Challa, with his royalty background, is someone who isn’t easily impressed by either Iron Man or Captain America. “He’s a guy who could go, 'Well I'm better at what you do than you. And I'm not gonna take either of your crap.’ It’s fun to have that guy show up after we watch the in fighting and squabbling over however many movies now - someone to come up and go, 'You don't matter. This is a country in Africa"
It's important to note the inclusion of Wakanda having a "Great place in the story" which I feel seals the fact that we will be seeing Wakanda in Civil War in some fashion. Also Chadwicke's emphasis that he is "A Prince Of Wakanda" and "A Black Panther" (as opposed to THE Black Panther) confirms that T'Chaka is alive and that for whatever reason T'Challa has been granted access to the Mantle.
Also for those who had lingering doubts that T'Challa would play second fiddle or be on Iron Man's team, they basically just shut that down once and for all. T'Challa isn't impressed with either Cap or Tony, he's out for his own reasons and motivation. This is in line with T'Challa's characterization in the comics, where he sons Iron Man regularly and can go toe to toe with Captain America. This is the same man who joined the Avengers with the intent being to spy on them and gather their weaknesses. T'Challa is for the good of Wakanda, everything is else is secondary (as it should be)
If we get just ONE scene of Tony Rambling off in his "Billionaire play-boy philanthropist" shtick and T'Challa literally gives him the
I'll stand up in the fukking theater and clap