7. Jocasta
Jocasta could be introduced in the Avengers sequel, initially as a villain, given that she was created by Ultron, the primary villain in the movie. Ultron created Jocasta as a mate for himself whilst he was in an abandoned aerospace research center in Nassau, Long Island, New York. Ultron based Jocasta on the mind-patterns of Janet Pym the Wasp and wife of his creator (in the comics, at least) Hank Pym. He also set about transferring Janet's lifeforce in to Jocasta in order to animate his robotic mate. The Janet aspect of this plot would have to be changed, given that she won't be introduced until after the Avengers sequel, and someone like Pepper Potts could be the base of the Jocasta idea. In the comics, upon learning that the lifeforce transfer would kill Janet (Pepper, or someone else, in the case of the potential movie version), Jocasta notified the Avengers of Ultron's plans and they ultimately managed to save her as a result which, in turn, left Jocasta as nothing more than an inanimate object. Pym (read Stark, for the case of the potential movie version) took possession of her for the purposes of study and, after a series of events that saw her revived, rejoining Ultron, then betraying him, she was taken by the Collector (giving her a Guardians of the Galaxy link as well). She was then saved by the Avengers and became a member of the team. She has the ability to integrate with computers, superhuman strength, durability and senses, optic cannons, the ability to generate force fields and the ability to create holographic disguises for herself. She's an extremely interesting character who could play a big part in the MCU if she was brought in to it, as she could eventually become a part of the Avengers.