Maybe they get stuck in an alternate universe where they are the only superheroes. They then have to figure out how to get back while also protecting this new world from the villains there. In this situation, they might learn more about each other's values and become closer friends than before.
Another plot could be that there is a mystery involving a powerful artifact. Iron Man uses his scientific knowledge to analyze it, while Captain America uses his leadership and combat experience to protect it from those who would misuse it. Along the way, they have deep conversations about power, responsibility, and friendship.
One interesting fanfiction could be where Captain America and Iron Man have to team up in an alternate universe to fight a new threat that combines their worst enemies' powers. It could explore their relationship in a different setting, highlighting their differences and how they overcome them to save the day.
A plot idea could be that Spider - Man discovers a hidden agenda of one of the sides during the civil war. He then has to decide whether to expose it or keep it to himself, as it could have far - reaching consequences for the superhero community.
Another idea is that there's a villain who can manipulate time and space. He sends Captain America to the Naruto world during a crucial time in Naruto's story, like right before a big battle. Captain America's leadership and battle experience help Naruto and his friends come up with a better strategy. And in the process, they learn from each other's values and fighting styles.
One plot idea could be an OC who is a scientist during World War II and teams up with Captain America. They could work on some top - secret projects together. Another is an OC who is a foreign agent sent to assess Captain America's abilities and ends up becoming his ally. Maybe an OC from the future who travels back in time to help Captain America prevent a major catastrophe that wasn't in the original timeline.
One possible plot reason could be a major ideological difference that escalates out of control. For example, they might have a huge disagreement over the use of a new technology or how to deal with a particular threat, and Iron Man in a fit of rage or desperation takes it too far and kills Captain America.
One idea could be to explore his origin story in more depth. Maybe he has a hidden family or a past trauma that shaped him into the man of iron. Another could be to put him in a different time period, like the future where technology has advanced even more and he has to adapt.
One possible storyline could be them realizing their deep emotional connection during a particularly tough battle. They start to rely on each other more than just as comrades, and slowly feelings of love and attraction develop. For example, Iron Man might be injured and Captain America stays by his side constantly, leading to a new level of closeness.
One idea could be inserting yourself as a new recruit in the Howling Commandos. You could have a special skill, like being an expert in decoding enemy messages. Then, you get to interact with Captain America and the team, facing challenges together and forging strong friendships. Another plot could be that you are a scientist in the same era as Steve Rogers. You accidentally discover a new element that could enhance the super - soldier serum. This draws the attention of both Captain America and the bad guys, leading to a race to protect or steal your discovery.
A plot could involve an alternate universe where Bucky was never captured by Hydra. He and Steve continue their adventures together, fighting different villains. Or perhaps a story where the Winter Soldier is sent on a mission to assassinate Steve, but at the last moment, his true self starts to break through and he has to fight against his programming to save Steve.
One interesting plot idea could be that Captain America and Agent 13 are on a mission to retrieve a powerful and dangerous artifact. Along the way, they face many challenges and have to rely on each other's skills. Agent 13 uses her espionage skills to gather intel, while Captain America uses his combat abilities to fight off enemies.