One common theme is loyalty. Their loyalty to each other is often a central part of the stories. They've been through so much together and would do anything for one another.
In comics, while Bucky has taken on the mantle of Captain America at certain points, it's not a permanent or default status. Steve Rogers is the original and most commonly associated Captain America.
A fanfiction could focus on an alternate universe where Bucky becomes Captain America instead. He has to deal with the same ideals but in his own unique way, while also having to face Steve Rogers in a different role, perhaps as an advisor or a reluctant ally. This would create a lot of interesting character dynamics.
Sure, Bucky has been Captain America in the comic universe. This happened after specific storylines and events that led to the passing of the mantle or under special circumstances.
One common theme is the struggle to overcome physical injury. Captain America is often depicted as superhuman, but in these fanfictions, his human side shows through when he has to deal with pain and the process of healing.
One common theme is the clash of different ideologies. For example, when Captain America crosses over with a dystopian universe, his values of freedom and justice often conflict with the oppressive regimes there. Another theme is the exploration of friendship. When he meets characters from other franchises, they have to learn to trust and work together.
One common theme is the exploration of how Captain America deals with the unexpected situation of pregnancy. It could be about his struggle to balance his superhero duties with his newfound role as a potential father.
A common theme is empowerment. Since it's a female version of Captain America, the stories often focus on her breaking gender barriers and proving herself in a male - dominated superhero world. For example, she might have to fight against sexist villains or deal with male characters who underestimate her at first.
Another theme is loyalty. Many OC fanfictions explore the loyalty that the OC develops towards Captain America and his ideals. This can be tested in various situations, like when facing a powerful enemy or when having to make a difficult choice between different loyalties. It adds depth to the relationship between the OC and Captain America.
Family is a very common theme. Captain America, who has lost so much in his own life, often forms a new kind of family unit with the baby in these stories. Protection is also big, as he uses his skills to keep the baby safe from various threats.
One common theme is sacrifice. Captain America might have to sacrifice some of his superhero duties to take care of his pregnant partner. Another theme is protection. He will do everything to protect his partner and the unborn child from any threats. And there's also the theme of family values. It shows how important family is to him even in the middle of his superhero life.