To be quite honest, it doesn't really matter who is in a James Cameron movie. Even fantastic actors—like Kate Winslet and Leo DiCaprio—cannot save his films from corny dialogue and predictable plots. And yet we go see James Cameron's movies anyway, because the guy has mastered and then reinvented the technical aspects of film-making again and again.
So does it really matter who is in Avatar 2? Not really. The film could star the guy who cleans James Cameron's pool and it would still make $500 million at the box office. That being said, the prospect of Arnold Schwarzenegger reuniting with Terminator director James Cameron to shoot some aliens does sound enticing. And according to Latino Review, this is happening.