In the movie, his sacrifice was more of a physical fight - related one. But in a fanfic, it could be more of an emotional or mental sacrifice. For example, he could sacrifice his own sanity to power up a device that saves everyone. He might go into a state of eternal madness while his friends watch helplessly, which is very different from the movie's more action - based sacrifice.
His sacrifice in a fanfic could be more personal. In the movie, it was for the general good of the universe. But in a fanfic, he could sacrifice himself specifically for one person, like Natasha. He might have a deep - seated love for her and give up his life to bring her back or protect her in a way that is much more focused on a single relationship rather than the overall fate of the world like in the movie.
Well, in the movie, Banner's sacrifice was sort of a heroic last - stand kind of thing during the big battle. In a fanfic, his sacrifice could be a pre - emptive one. He could sacrifice himself before the big battle even starts. Maybe he discovers a way to prevent the whole catastrophe but it requires him to give up his life immediately. This would change the whole narrative flow compared to the movie.