The Winter Soldier doesn't become Captain America in the regular comic storylines. It's Steve Rogers who holds that position. But comics are full of surprises and alternate universes, so there could be special instances where this happens in some less common or alternate versions.
In 'Captain America Winter Soldier' fanfics, the theme of trust is often present. Bucky has to learn to trust again, especially Steve. And Steve has to trust that Bucky can overcome his past. Another theme is the fight against evil. Even though the main threat in the movie was Hydra, fanfics might introduce new villains or continue the fight against the remnants of Hydra. Also, the theme of family is there in a way. Steve and Bucky are like family to each other, and fanfics might explore how they protect and support each other like a family would in difficult times.
There are also some specialized fanfiction forums. For example, Reddit has sub - forums where fans might share their own Captain America Winter Soldier fanfiction or recommend good ones they've read. You can join these communities and explore the available fan - made stories.
Another option is Wattpad. While it has a wide range of content, there are definitely some interesting 'Captain America: The Winter Soldier' fanfictions on there. Some writers may post exclusive content on Wattpad, so it's worth checking out.
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.
Black Widow would be a great character to further involve. She has a complex past and her skills could add more depth to the story. Sam Wilson, too. His friendship with Steve could be explored more, and he could play a key role in helping Bucky adjust to the modern world. Another good addition could be Agent 13. She could be involved in some sort of undercover mission related to the Hydra secrets that are still out there.
There's a fanfiction called 'Redemption of the Winter Soldier'. In this story, Bucky tries to make amends for his past actions as the Winter Soldier in a more complex and detailed way than in the movie. It's really engaging as it shows his internal struggle. Also, 'Steve's Choice' is a story that explores the choices Steve has to make regarding Bucky and his role as Captain America, and it's a great read.
One great 'Captain America Winter Soldier' fanfic is 'The Winter Soldier's Redemption'. It delves deep into Bucky's struggle to regain his humanity after being the Winter Soldier. Another is 'Cap and Bucky: Reunited', which focuses on the strong bond between Steve and Bucky during their post - Winter Soldier days. And 'Winter's End' is also a popular one, showing how Bucky adapts to the modern world while dealing with his past traumas.
One way is to read reviews or summaries of the fanfiction before starting. This can help you find the ones that are more likely to be interesting to you. For example, if you prefer action - packed stories, look for reviews that mention lots of fight scenes in the 'Captain America: The Winter Soldier' fanfiction.