5. Loki
Thor: The Dark World ended with Loki still at large and indeed sitting in a position of power from which he could cause all kinds of trouble. The big reveal that he hadn't died came as no real surprise, since he is an incredibly popular character that Marvel want to do more with and that is the main reason that he could have a part to play alongside Ultron in the Avengers sequel. Anyone wanting to take the Avengers down will be okay in Loki's book and a team-up with the robot would be a very interesting prospect that could be the ideal way to plant the seeds for a live action Masters of Evil in the future. Loki spent that last Avengers playing the role of flunky to Thanos. This time around, approaching Ultron for a more equal team-up might give him a chance to defeat the Avengers once and for all.