Loki was the first major villain to make his mark in the Loki now has his own confirmed Disney+ spin-off show.
Marvel movies have always had frequent criticisms in the villain department, mostly boiling down to a lack of depth, a lack of character development, or a lack of nuanced motivation. Of course there are a couple exceptions, with Erik Killmonger and Thanos himself springing to mind, but the overwhelming majority of MCU villains have been morally corrupt villains killed off without the chance to blossom into something new. With Marvel Studios recently re-acquiring the libraries of characters like the X-Men and Fantastic Four, the MCU has the chance to reconcile these complaints by introducing morally complex characters such as Doctor Doom and Magneto.
Aside from introducing new characters though, there's another way that the MCU can fix their frequent villain shortcomings: bringing them back. Most of the villains in the MCU have been one-off appearances, either dying or being permanently imprisoned by the end of the film. Having certain villains return in multiple movies gives the filmmakers involved a chance to build on their characterization in unique ways, and as of right now there are several MCU villains that have earned this treatment.
Baron Zemo
Introduced in MCU's version of Helmut Zemo is one of the most morally complex and interesting villains introduced on-screen so far. Birthed as a direct result of the Avengers' irresponsibility and lack of ability, Zemo vowed to destroy the Avengers from the inside out after their intervention in Sokovia resulted in the death of his family. The strategic experience and tactical brilliance that he received as part of the Sokovian Armed Forces allowed him to prey on the interpersonal relationships between characters like Steve Rogers and Tony Stark, and as a result, Zemo was able to splinter the Avengers and make them susceptible to Thanos' invasion of Earth.
However, fans already know that the MCU isn't done with him. Zemo's already been confirmed to appear in The Falcon and the Winter Soldier in some capacity, and while it's entirely possible that he'll simply be a villain, the way Civil War concludes his character arc feels like there's much more in store for him in the future. In the comics, Zemo is a character who has flirted with reformation multiple times, even leading his own roster of the Thunderbolts for a time. Hopefully fans will get to see him in all of his moral nuances (along with his iconic purple mask) the next time we see him on-screen.
Karl Mordo
When Doctor Strange first announced that Chiwitel Ejiofor would be playing a reimagined version of Baron Mordo from the comics, fans already expected his cinematic counterpart to be pretty different. In a lot of ways, Karl Mordo did resemble the original villain from the comics: a former student of the Ancient One, harboring a complex rivalry with Stephen Strange. The big differences are in how he becomes a villain: instead of a thirst for power or a desire to become stronger than his archnemesis, the MCU's Mordo is corrupted due to his rigorous adherence to tradition and his blind faith in the Ancient One. When she fails him, he has no choice but to view magic as naturally corrupt.
This sets him on a dark path, one that's totally open for any number of potential reappearances. While it would make a ton of sense for him to reappear in Mordo return in a villainous role only to see him once again redeemed to the side of good; Strange may unintentionally awaken another villain along the lines of Dormammu or Mephisto, forcing the two former allies to once again work together.
Taskmaster
While audiences haven't gotten a chance to meet Taskmaster just yet, they will as soon as Black Widow finally drops in theaters. In the comics the character is notorious for having a wide range of targets in the superhero community, terrorizing characters such as Daredevil, Captain America, and Black Widow. Taskmaster is a character that fits comfortably within the black and white supervillain trope, as he's originally simply a hired assassin with the power to duplicate the fighting techniques of anyone he encounters. This means that as long as he sticks around post-Black Widow, there's a major chance that Taskmaster could show up anywhere, including the Disney+ shows or future movies.
Taskmaster has frequently been a threat to the Avengers as a whole, so he could even show up in a major team-up movie, working either alone or as the muscle for a bigger mastermind behind the scenes.
Ultron
Unlike the other villains on this list, Ultron is the one character we've seen "die" on-screen so it might be a stretch to see him return in the future. However, Ultron in the comics has escaped certain death multiple times before, and the MCU version of the character has already shown us that he's capable of transferring his consciousness to other bodies. This is a very easy explanation to lean on in the event that Marvel Studios wants to use Ultron again in the future. Knowing how much oversight he had in his first appearance, it would make a lot of sense to reveal that he had another proxy body waiting to be used in the event that his final form was destroyed.
His status as an Avengers-level threat means that he could be used again in a variety of ways, from once again becoming a major villain to returning for a role in a solo film. His relationship with his creator, Tony Stark, could take on a very interesting dynamic now that Stark is dead in the MCU. Seeing the Earth trying to move on without its greatest protector could be an opportunity for Ultron to once again try and purge humanity due to their warmongering and violent nature. On top of this, there's a lot of unresolved story potential to be mined in regards to Ultron creating Vision and the relationship that the two of them share.
Dormammu
One of the most powerful magic s in the entire Marvel Universe, Dormammu has been a consistent threat for not just Doctor Strange, but the entire universe as a whole. The ruler of the Dark Dimension, Dormammu has frequently tried to lay siege to Earth and its citizens, hoping to enslave them and merge the planet with his own dimension. The immortal villain appeared briefly at the end of the first Doctor Strange, trapped in a never-ending time loop by the Sorcerer Supreme, but there's absolutely no way that Marvel Studios hasn't at least considered bringing him back for a round 2 with everyone's favorite magician.
Dormammu is such a massive threat because he frequently outclasses Doctor Strange in of sheer, raw power, forcing the hero to find more inventive ways to deal with him. He's a villain who, if not kept in his own realm, could pose a major challenge to not just Doctor Strange but the entire team of Avengers. This sets him up perfectly to serve as a major villain for the next generation of Avengers, the new team and status quo teased by the ending of Avengers: Endgame. While Thanos' defeat granted Earth a much needed moment of respite, there are still dozens of threats lying in wait in the MCU, ready to take on the Avengers at their weakest.
