First, start with his past. Show how his experiences as the Winter Soldier have made him who he is at the start of the fanfic. Then, gradually introduce the influence of the reader's daughter. Maybe she shows him kindness, and that starts to chip away at his cold exterior. As the story progresses, he should start to question his old ways and beliefs. By the end, he could have made a significant change, like becoming more open and less of a weapon, more of a protector with a new sense of purpose.
One character that would be interesting to include is Isaiah Bradley. His story is so rich and tragic, and his connection to the super - soldier program could bring a whole new layer of depth to the fanfic. Additionally, Agent 13 could be included. She could be involved in some sort of political intrigue that Falcon and Winter Soldier have to navigate through. And Redwing! Sam's trusty drone could have its own sub - plot, like getting hacked and Sam having to rescue it.
One common theme is the clash of cultures. Naruto's world is full of ninjas and chakra, while the Winter Soldier's world has advanced technology and a different kind of warfare. Another theme could be redemption. Just as the Winter Soldier has a past to atone for, Naruto also has his own demons to face, and in fanfics, they might help each other in that journey.
Female characters in female Winter Soldier fanfic usually possess unique qualities. Many are independent thinkers. They are not afraid to stand up to the Winter Soldier when necessary. Some are healers, either literally as doctors or metaphorically as those who heal his emotional wounds. They bring out different sides of the Winter Soldier, like his more human and vulnerable side. For instance, in some fanfics, a female character might be an artist who uses her creativity to help him see the world in a new light. Moreover, they are often the driving force behind changes in the story's plot as they interact with the Winter Soldier.
There are authors who are great at writing action - packed 'Black Widow and Winter Soldier fanfic'. These are the ones who can describe the fight scenes vividly, making it feel like you're right there in the middle of the action. They also know how to balance the action with the character development. Another type of great author is the one who delves deep into the emotional side of the characters, exploring their feelings and relationships in a very nuanced way.
One common theme is redemption. The Winter Soldier has a dark past, and often in these fics, the OC plays a role in helping him find redemption. For example, through their friendship or love, the OC encourages Bucky to face his demons.
He could interact in a protective way. For example, if it's a small animal like a rabbit, he might build a safe place for it. His military training would make him good at creating a secure environment.
For 'dark winter soldier fanfiction', Falcon could be an interesting character to pair with the Winter Soldier. Falcon has his own set of skills and a strong moral compass. He could help the Winter Soldier navigate the modern world and his newfound freedom. Also, Scarlet Witch would be a fascinating pairing. She could potentially help him deal with his traumatic memories through her powers. And Bucky could also be paired with Doctor Strange. Strange could use his magic to help Bucky understand and control the dark forces within him.