Marvel Cinematic Universe: 10 MIA Villains Who Need To Return

4. Ultron

Last Seen: Avengers: Age of Ultron

Despite appearing in only a single film - 2015's first Avengers sequel - Ultron has left quite a mark on the MCU. Aside from being responsible for the death of Quicksilver, his actions in Sokovia resulted in a great many side effects, like sparking Helmut Zemo's hatred of the Avengers and giving birth to the Sokovia Accords, all of which weakened the Avengers leading up to Infinity War.

It's unfortunate that the actual age of Ultron lasted mere days, however, as the villain himself - unforgettably portrayed by James Spader - was an engaging presence. Of course, one of the stipulations set forth at the outset of this list was that the villains on it must have been alive the last time saw them, and though it seems that Ultron may be "dead," there's enough ambiguity about whether he may still exist somehow, such as returned to the Mind Stone, that it's impossible to not include him.

His roots in the comics have demonstrated time and again that he never stays down forever, and having him return in some form - even if for a surprise appearance in Avengers: Endgame, which seems to be revisiting past events of the franchise - would be great, particularly for more of Spader's performance, as Ultron and his motivations will have undoubtedly evolved in the time that's slipped by.