Maybe in the fanfic, Captain America is accidentally transported to the DC universe and the Justice League discovers him. They initially mistrust him because he's from another world, but as they see his unwavering moral compass in action, like when he saves a group of civilians from a super - villain's attack without hesitation, they start to accept him as one of their own.
One plot idea could be Captain America joining the Justice League temporarily. He could bring his military tactics and sense of honor, and have clashes and then reconciliations with Batman over leadership and strategy. For example, when facing a powerful alien invasion, Captain America might advocate for a more direct, all - out attack while Batman prefers a more stealthy, long - term plan.
In a fanfic, Captain America could fit into the Justice League by being the voice of the common man. The Justice League often deals with big, cosmic threats, but Captain America would remind them of the importance of protecting the ordinary people on the ground. His experience in World War II gives him a unique perspective on the cost of war and the value of peace, which could influence the Justice League's decisions when it comes to using force and resolving conflicts.
One idea could be a story where Batman has to lead the Justice League in a fight against a powerful new villain who can manipulate time. This would put Batman's strategic skills to the ultimate test as he tries to coordinate the different powers of the League members. Another plot could be that a member of the Justice League is secretly under mind control and Batman has to figure out who it is before they cause too much damage.
One idea could be an OC with the power to manipulate time joining the Justice League. They could accidentally create a time rift during a battle, and the League has to fix it while also dealing with the OC's insecurities about their powers.
One idea could be that the Young Justice team has to prove themselves to the Justice League during a major alien invasion. The League initially doubts their abilities but as the battle progresses, they see the unique skills and teamwork of the young heroes.
It could be really interesting. Steve Rogers' sense of justice and leadership would blend well with the Justice League. His military tactics could help in planning battles against powerful villains. For example, in a fight against Darkseid, Steve might come up with a strategy to use the different powers of the League members effectively, like having Flash create distractions while Superman goes for a direct attack.
One plot idea could be that Captain America and Scarlet Witch team up to face a new, powerful villain who has the ability to manipulate time. They have to combine Captain America's combat skills and leadership with Scarlet Witch's reality - warping powers to defeat the foe.
One plot idea could be that after being brainwashed, Captain America is made to believe his old friends are enemies. So he starts targeting the Avengers. Another could be that he is brainwashed to carry out a secret mission that goes against his moral code, like stealing important artifacts for a shadowy organization. And he has to struggle to break free from the brainwashing when he starts to have flashes of his true self.
One idea could be exploring the backstories of the lesser - known members in more detail. For example, Martian Manhunter's life on Mars before coming to Earth.
One idea could be a time - travel fanfic where Captain Marvel and Captain America team up in a different era. Maybe they end up in the 1920s and have to deal with a Hydra - like threat that's starting to form early. Another could be a fanfic where they swap powers for a day and have to learn to adapt to each other's abilities. For example, Captain America trying to control Captain Marvel's photon blasts and Captain Marvel having to rely on just her combat skills like Cap usually does.
One plot idea could be that Black Widow and Captain America are on a secret mission together in a war - torn country. They have to rely on each other's skills to infiltrate an enemy base and retrieve a dangerous weapon. Another could be a story set in the past, where they first meet during their training days and form an unlikely friendship that develops over time. Maybe a plot where they are both framed for a crime they didn't commit and have to work together to clear their names while also uncovering a larger conspiracy within S.H.I.E.L.D.