Maybe a male OC with the power of telepathy and teleportation. He was raised in a secret organization that trained him to be an assassin. However, he rebels against them and decides to use his powers for good. When he crosses paths with the Avengers, he has to overcome his past and learn to work within a team environment.
One idea could be a male OC who has the power to manipulate time. He could be from a different dimension where the Avengers failed, and he comes to this universe to prevent the same fate. He might initially be distrusted by the Avengers but gradually prove his worth as he helps them defeat a powerful new enemy that was also a threat in his own dimension.
First, give him a clear backstory. For example, if he has superpowers, explain how he got them. Was it an accident, genetic, or something else? Second, give him flaws. He can't be perfect. Maybe he's too impulsive or has a fear of heights which can create interesting situations in battles. Third, give him relationships. He could have a family member he's trying to protect or an old friend who has turned bad.
One great idea could be Ben using his Omnitrix to transform into an alien that has a power similar to one of the Avengers. For example, turning into an alien with super strength like the Hulk and having a friendly competition with him. Another idea is that the Avengers could mistake Ben for a threat at first because of his alien forms and then they realize he's actually on their side. They could then team up to fight a common enemy, like a powerful villain from the Ben 10 universe that has somehow made its way into the Avengers' world.
One idea could be that Spider - Man has to use his unique abilities to solve a mystery within the Avengers' headquarters. Maybe there are some strange energy signatures that only his spider - sense can detect.
One idea could be that Naruto is somehow transported to the Marvel universe where the Avengers are. He could use his ninja skills, like his shadow clone jutsu, to confuse the villains that the Avengers are facing. Maybe he teams up with Iron Man and they combine their tech and ninjutsu to create some really cool weapons or defenses.
One great example could be a fanfiction where Falcon takes on a leadership role in the Avengers. Maybe after some initial doubts from the team, he proves his worth by devising a brilliant strategy to defeat a new enemy. This could show his growth from being a sidekick - like figure to a true leader within the Avengers.
One idea could be that Spider - Man has to use his unique street - level skills to help the Avengers in a mission where they are facing a villain who can manipulate technology in a city. His agility and quick thinking can give the Avengers an edge in the narrow alleys and on top of skyscrapers.
A great plot could be that the male OC has a past connection to one of the main Avengers villains. For example, he was experimented on by the same organization that created the Hulk. This leads to a complex situation where the Avengers don't know if they can trust him. Or, the male OC could accidentally time - travel and change a small event in the past that has major consequences for the present - day Avengers, and he has to fix it.
One plot idea could be an Avenger taking in an OC (original character) who has unique powers but is scared to use them. The Avenger, like Thor, could teach the OC about courage and how to control their powers. Another idea is an OC who is a tech genius and gets adopted by Tony Stark. They work together on new Iron Man suit designs. A third idea is an OC with a dark past being taken in by Black Widow. She helps the OC face their demons.
One idea could be an OC who has the power to manipulate time like Pietro. They could team up with Pietro to save the Avengers from a time - trapped enemy. The OC might be from a different dimension where time works differently, adding an interesting dynamic.
One plot idea could be that the female OC supersoldier is sent on a mission to retrieve a powerful artifact before the bad guys get it. Along the way, she has to team up with some of the Avengers she doesn't trust at first. Another idea is that she discovers a dark secret about the supersoldier program she was part of that has ties to an enemy of the Avengers. Or she could be framed for a crime she didn't commit, and the Avengers have to decide whether to believe her or not while she tries to clear her name.