Perhaps there is a common enemy that forces Captain America and the RWBY characters to work together. This enemy could be a powerful being that has the ability to manipulate both the technology in Captain America's world and the Dust in RWBY's world. The combined forces of the two universes would have to find a way to defeat this enemy and in the process, learn from each other's strengths and weaknesses.
One possible plotline could be that as Captain America turns into a girl, she has to deal with the sudden change in her physical form while still having the same strong will and sense of justice. She might struggle with how others in the Avengers or the world perceive her now. Maybe she has to prove herself all over again, not just as a female version of Captain America, but also in a male - dominated superhero world.
One idea could be that Captain America is somehow transported to the RWBY universe. He could team up with Team RWBY to fight against the Grimm. His shield would be a unique weapon in that world, and his leadership and combat skills would be highly valuable.
One possible plotline could be that a new character enters the scene, perhaps a powerful and attractive superhero or agent. Captain America starts showing interest in this new person in a friendly way, but Black Widow misinterprets it as something more. She becomes jealous and starts acting cold towards Cap. Meanwhile, Cap is oblivious to her jealousy at first. As the story progresses, they have to work together on a mission and gradually realize their true feelings for each other through various challenges and misunderstandings.
Perhaps in the fanfic, Captain America's shield could be modified with Dust technology in the RWBY world. He might also have to adjust to the different fighting styles and powers of the RWBY characters. For example, learning about Aura and Semblances and how they can be used in combination with his own abilities in combat.
Another plotline might involve characters who were on the sidelines in the movie having a more prominent role in the fanfiction. For example, a minor character could have the key to resolving the conflict between Captain America and Iron Man. They could use their unique perspective or abilities to bridge the gap between the two factions.
Imagine a story where Captain America and Black Widow are the last remaining Avengers in a world that has been taken over by a powerful and evil force. They have to start a resistance movement from scratch. They train new recruits, gather resources, and plan strategic attacks. Their relationship would be tested as they face numerous setbacks and difficult decisions in their fight for freedom.
One interesting plotline could be them teaming up against a new, powerful villain that neither the tech of Stark nor the leadership of Cap could easily defeat alone. They have to combine their skills and trust each other completely.
Some fanfictions have a plotline where Spiderman is actually a double agent. He pretends to side with Iron Man but is secretly working with Captain America to expose some hidden agenda. This adds a whole new layer of intrigue to the story.
One common plotline is that after getting hurt, Captain America has to deal with physical and mental recovery. He might struggle with self - doubt, thinking he's not as strong as before. Another is that his team has to step up to protect him while he's injured, which could lead to character development for the other Avengers. For example, in some fanfictions, Bucky takes on a more protective role over Steve.
Another plotline could be about Sharon helping Captain America adjust to the modern world. Maybe she takes him on a tour of modern technology and they bond over their shared love for justice. They could also be involved in a mystery where they have to use their different perspectives (Sharon's modern knowledge and Captain America's old - fashioned values) to solve it. This would focus on their friendship and mutual respect.