The Hulk in Endgame doesn't fully match the comics. For instance, his power levels and character development might vary. Also, the events and circumstances surrounding him might be changed to fit the movie's narrative.
Another fanfiction idea could be a prequel to her role in Avengers: Endgame. It could show what she was doing in the cosmos just before she received the call to return to Earth. Maybe she was facing a different threat that was related to Thanos in some way, and this backstory could add more depth to her character in the movie.
One possible fanfiction could involve Spider - Man and Captain Marvel teaming up during the Endgame battle. Maybe Spider - Man uses his agility to distract the enemy while Captain Marvel blasts them with her powerful energy. They could also have some fun banter, like Spider - Man making a joke about how Captain Marvel can fly and he has to swing around.
The separation of the Hulk and Banner varies in Marvel comics. Sometimes, external forces or internal conflicts cause them to split, but it's not a consistent occurrence throughout all comics.