The story of 'Captain America: Winter Soldier' is complex. It mainly revolves around Steve Rogers adjusting to the modern world. Hydra has secretly infiltrated S.H.I.E.L.D. The Winter Soldier, Bucky Barnes, who was Steve's old friend, has been brainwashed by Hydra to be a deadly assassin. Steve discovers the truth about S.H.I.E.L.D. and Hydra's plot. He teams up with Natasha Romanoff to fight against this conspiracy and also tries to save his friend Bucky from the control of Hydra.
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.
The plot mainly revolves around Captain America's discovery of a conspiracy within S.H.I.E.L.D. He teams up with the Winter Soldier, who has a complex past tied to Cap. There are intense action sequences as they fight against Hydra agents who have infiltrated S.H.I.E.L.D.
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.