Moon Knight has been a long-time favorite for Marvel fans and is presently in development for Disney+. Marc Spector was a soldier who perished and was brought back to life by the Egyptian god of vengeance, Khonshu. Additionally, due to traumatic events of his youth, Spector suffers from dissociative personality disorder.
This would be one of the MCU’s first superheroes with a mental illness that could be talked about on screen, providing a much-needed form of representation in an otherwise barren field of radically “perfect” heroes.

After receiving a blood transfusion from her cousin Bruce Banner, Jennifer Walters undergoes a similar transformation to the OG Hulk. Her transformation is permanent, similar to Professor Hulk in Avengers: Endgame. Over her comics history, she’s been both a supermodel and a skilled lawyer, often acting as legal counsel for the Avengers.
In addition to the Avengers, She-Hulk has been a member of the Defenders, Heroes for Hire, the Fantastic Four, and S.H.I.E.L.D., and she’s a founding member of A-Force.

Presently on ice after rejoining Marvel Studios at the House of Mouse, the Fantastic Four are a bit of a weird team to talk about when discussing potential Avengers for future MCU properties. While all four of them have been a part of the Avengers at one point or another, it is very rare that all four of them are Avengers at the same time.
Kevin Feige already stated at the 2019 Comic-Con International Marvel panel that Fantastic Four and mutant-related properties are in very early development, meaning that they likely won’t make their debut anytime soon.

Namor the Sub-Mariner is the one comics property that Warner Bros. and DC beat Marvel to the punch on when it came to film distribution; he and Aquaman are incredibly similar characters. They’re both rulers of some version of Atlantis and have both Atlantian and human heritage.
Chances are Marvel Studios has not introduced Namor to the MCU for this very reason, however, as of Avengers: Endgame, people have been theorizing that we may have gotten our first hint at his existence in the form of a report Okoye gives Natasha about a deep-sea earthquake off the coast of Africa. Considering Marvel’s history with peppering Easter eggs for future titles in current films, this would totally fall in the same category as the Stephen Strange name drop from Captain America: The Winter Soldier.

Spider-Woman is surprisingly not the female equivalent of Spider-Man. In the ’70s, Marvel created the character when two parents tried to cure their daughter, Jessica Drew, after long-term exposure to Uranium. Her father, geneticist Dr. Jonathan Drew, injected her with a serum made from the blood of spiders in the hope that their regenerative properties would act as an immunization to further damage. 
When Jessica was a teen, she briefly joined Hydra, which tried to train her as a hitman for the organization. Once she realized the nature of the organization, she escaped, slaying her mentor and eventually joining S.H.I.E.L.D.

Nova and the accompanying Nova Corps are basically Marvel’s answer to the Green Lantern Corps, and they’re arguably more effective. Making their home on Xandar, the Nova Corps has already been established in the MCU, as seen in Guardians of the Galaxy, and was both snapped and snapped back to reality in Avengers: Infinity War and Endgame, respectively.
Marvel has given the Nova mantle to a few characters throughout the comics and we have already seen two potential mantle-bearers in Guardians of the Galaxy in both Irani Rael, played by Glenn Close, and Rhomann Dey, played by John C. Reilly. While both of these less conventional casting choices would be incredible, there is a strong chance that if Marvel Studios ever makes a Nova film or limited series that they would go the Samuel Alexander route and cast a young actor in their teens or early twenties.

Originally played by Adrianne Palicki on Agents of S.H.I.E.L.D., Dr. Bobbi Morse, AKA Mockingbird, is one of S.H.I.E.L.D.’s best and brightest. A biologist, Morse originally joined S.H.I.E.L.D. when they were trying to recreate the Super-Soldier Serum that was used to create Captain America back in the ’40s. Eventually, she was trained as a spy given her knack for espionage and languages and eventually gained enhanced physical abilities due to testing of the aforementioned Serum.
She was once married to Hawkeye, which could be a great way of bringing over Palicki’s brilliant portrayal of the character to the big screen.

Fresh off of his onscreen debut in Spider-Man: Into the Spiderverse, Miles Morales’s Spider-Man is one of the freshest additions to the Avengers roster, and with good reason. Although Miles’ existence in the MCU was hinted at in Spider-Man: Homecoming, we have yet to see the hero pop up in any of the films himself. The Avengers had already been around for a long time by the time Miles got his powers so it makes sense that he’d be held off for a bit – especilly since Peter Parker has only recently become an Avenger himself.

Ms. Marvel

Arguably the most popular character to come out of the Marvel Now! titles (alongside Kate Bishop/Hawkeye) is Kamala Khan’s Ms. Marvel. Kamala’s Ms. Marvel holds the honor of being the first Muslim character to ever headline her own comic book, and it takes the character into areas other superheroes haven’t really delved into before.
Themes of defying the teachings of faith for the greater good coupled with self-discovery and acceptance after a traumatic event would add a fresh amount of depth to the Avengers roster.

Captain Britain is not a weirdo international version of Captain America; he is literally more magical. Brian Braddock was a graduate student in London who was run off the road while driving his motorcycle. On the brink of losing his life, he was approached by Merlin with an offer: live and become Britain’s next hero or perish right here. He accepted the offer and was gifted an amulet and a staff which granted him enormous powers of superhuman agility and strength, flight, and regeneration.
Captain Britain would fit in very nicely with Black Knight, who is slated to be added in The Eternals. Considering the now heavy shift into the mystical arts that Marvel Studios is taking with its upcoming properties and the fact that there is already a Merlin that was introduced in Doctor Strange, Captain Britain could be making an appearance sooner rather than later.