8. Ultron & Arnim Zola
Although the Red Skull was the major villain in Captain America: The First Avengers, Arnim Zola was just as memorable a character and it would be great to see him again. He is expected to reappear in Captain America: The First Avenger next year, though whether that will only be in flashbacks is unknown. His natural life would be long over in the present day (given that he was getting on a bit during World War II), but it would be great to see him as a cyborg (like he became in the comics) and, if that turns out to be the case, I think that a team-up between him and Ultron could be very interesting. As a major player in the development of HYDRA as a truly global threat, it is not unfathomable that they might have wanted to preserve him in cyborg form and, given that I genuinely think that HYDRA could be one of the entities Ultron seeks out for assistance, an encounter between the robot and (potential) cyborg would be cool - having them developing weapons and upgrading Ultron together would be great. It could lead to Zola potentially working as a weapons developer for an MCU Masters of Evil team in the future, with Ultron among its ranks.