The Panther movie would most likely get made first.
In the books, he's kind of a reserve Avenger. When Marvel decided to raise his profile a few years ago, they made him a more regular Avenger and featured him in more big storylines. They even used him and Storm to take the place of Reed and Sue Richards in the Fantastic Four for a while.
I wouldn't expect for them to shoehorn him into the Avengers as he has a big enough story to have his own movie and if they want to, they can put him in later.
