You know, this *could* very well point towards Black Panther being in AOU after all. Not a full-blown role, mind you, just a cameo; and he probably wouldn't join the Avengers until the end of the movie --- or in a later film; and it could be as T'Chaka instead of T'challa. But here's how Marvel could effectively "cheat" BP into the film:
....show Black Panther in costume, but don't take the mask off. That way, *anybody* with a good physique could act as a stand-in/placeholder BP in this movie, while still giving Marvel plenty of time to continue the casting search for the *right* T'challa to be unmasked in a later movie.
It just stands to reason that if Klaw is in this, and vibranium is in this, then it stands to reason that Wakanda is in this, too. (Maybe Marvel has been throwing red herrings about the whole Johannesburg setting --- yeah, it was definitely *filmed* there, but maybe it's supposed to represent Wakanda instead of South Africa, only Marvel doesn't want anyone to know that yet.) And if all the above is true, it also stands to reason that Black Panther plays a small role.