As the world of the MCU continues to grow, more and more characters tend to have recurring roles, where the added screen time allows them to be fleshed out over the course of multiple movies, while also creating casts that are not solely centered around the hero and their villain.
Some characters have not had this experience, though, making their debut only to never be seen again, with much left unexplored and unsaid. These 'one-off' appearances concern MCU movie characters, rather than those who have only appeared in one TV show.
Ava Starr / Ghost
Introduced as an antagonist in Ant-Man and the Wasp, Ava Starr, also known as Ghost, is a woman who gained powers 0f intangibility after going through quantum mutation. In a change of pace from most MCU movies, Ava's character is not a traditional villain. Her goals conflict with Scott and Hope's, but she does not have evil motivations.
At the resolution of the movie, she goes into hiding alongside her surrogate father Bill Foster, but with help to manage her powers, there is potential for her return. This would make particular sense given her connection to the Quantum Realm, which will feature in Ant-Man and the Wasp: Quantumania.
Pietro Maximoff / Quicksilver
Pietro Maximoff's appearance in Avengers: Age of Ultron only skimmed the surface of his powers, and his sudden death in the movie seemingly put a halt to any chance of further exploring the character.
It seemed momentarily that Fox's version of the character had returned in similarities between Fox's Quicksilver and the MCU's exist, Pietro's origins really change the character. With the establishment of the multiverse, though, the opportunity for Pietro's character to have a future has reappeared, which could expand on the hero's story, and provide new conflict for Wanda.
Dr. Helen Cho
Genius Dr. Helen Cho's only appearance in the MCU was in Age of Ultron, where her Regeneration Cradle technology played a pivotal role in the creation of Vision, as well as - unwittingly - the schemes of Ultron.
Towards the end of the movie, Helen is seen touring the new Avengers facility but has not made an appearance in the MCU since. Given her incredible intellect and inventive capabilities, a return of the character could bring along new innovations with benefits for the world's populace, civilians and heroes alike, while also giving the audience the chance to learn more about the fascinating doctor.
Aaron Davis / Prowler
The appearance of Aaron Davis in Spider-Man: Homecoming was of great importance not only for the character's own sake, but in that it established the existence of another potential Spider-Man, Miles Morales, in the MCU.
There are plenty of Miles Morales stories that could be adapted for the MCU, and given Aaron's importance in his nephew's life, his return would be vital as well. Aaron's reappearance would also give him the chance to suit up as the thief Prowler, an alias he uses for alternative purposes in Homecoming.
Hela
An Thor: Ragnarok, and though she was seemingly killed at the movie's conclusion, paths for her return exist.
Not only does the existence of the multiverse make it easy for Hela to reappear, but the focus on such variants in Loki sets up a relevant story in which she could do so, even if this version were not played by Cate Blanchett. Hela's earlier life was touched on in Ragnarok, but the millennia-old goddess lived for ages that are left unexplored.
Irani Rael / Nova Prime
The first Guardians of the Galaxy was packed with memorable characters, but a somewhat underwhelming element of the movie came about in its depiction of the Nova Corps, and their leader Nova Prime.
In the comics, the Nova Corps is an intergalactic military force whose gain power from the Nova Force, with leader Nova Prime able to gain abilities including flight, energy manipulation and projection. They're also associated with the superhero Nova, whose absence in Guardians was noticeable. Nova Prime's return could enrich the Nova Corps onscreen depiction, and introduce new outer space heroes.
Xu Xialing
A recent addition to the MCU and breakout character from Shang-Chi and the Legend of the Ten Rings, Xu Xialing is Shang-Chi's sister, a woman with incredible martial arts abilities who takes over from her father as the leader of the Ten Rings at the movie's end.
Her decision to keep the Ten Rings alive rather than oversee its dismantling as she told her brother she would seems to be setting up Xialing for future appearances in the MCU, and her complex relationship to both the organization and her brother make the perfect foundation for forthcoming stories.
Morgan Stark
Morgan Stark, daughter of Tony Stark and Pepper Potts, became a part of the MCU in Avengers: Endgame, and though she appeared as a young girl, depictions of an older version of the character were also filmed, though they were ultimately cut.
Nevertheless, there is plenty of space for Morgan's character to be explored in the future, whether that is in unpacking her father's legacy, interacting with the next generation of superheroes or becoming one herself. As Disney+ shows Armor Wars will both deal with Tony's legacy, a path for Morgan's reappearance could be made clear.
Alexei Shostakov / Red Guardian
The Soviet Union's answer to Captain America, Alexei Shostakov made his MCU debut in Black Widow, donning the Red Guardian suit to help his surrogate family fight against the Red Room.
There are plenty of places Red Guardian could appear next in the MCU, whether that be in reuniting with Yelena or confronting his once-nemesis Captain America - though that mantle has now ed on to Sam Wilson. Alexei's humor brought a great levity to darker aspects of the Black Widow story, and his reappearance could continually be used in this way, while his many years as the Red Guardian leave much to be uncovered.
Adrian Toomes / Vulture
The first villain that the MCU Spider-Man faces down against, Vulture, appeared as Liz's father in Spider-Man: Homecoming, and ended up in prison at the movie's conclusion.
In a perhaps unexpected turn of events, the character's gratitude for being spared by Spider-Man meant that he did not reveal the teen's secret identity to his fellow inmates. Given the events of Spider-Man: No Way Home though, Vulture could now potentially return without having this sense of duty to the hero, showcasing more of the character's cruelty.
