4. Wasp
Janet van Dyne is a founding member of The Avengers with the power to alter her size at will. When at miniature size, a pair of insect wings sprouts from her back allowing her to fly. She can generate bio-electric energy blasts from her hands called stingers. All of these powers come courtesy of Hank Pym, her long time love interest and ex-husband. Although Wasp is not a scientific genius like Pym, she has been an invaluable member of The Avengers. She is a skilled strategist, very intuitive, and often works out situations and solutions faster than other Avengers. She is very brave, willing to head straight into battle with villains much more powerful that herself. She has acted as leader of the Avengers on a number of occasions. Pym called Wasp the greatest superhero he had ever known.
Why should we expect her? Joss Whedon revealed that early drafts of The Avengers script had Wasp included rather than Black Widow. In addition, Whedon has said that he would like to add more female heroes into The Avengers 2. Wasp is also likely to be a significant character in the 2015 Ant-Man film. All of these factors present a compelling case that Wasp will appear as an Avenger in film.