In the fanfic, it could be that Peter's illness is a side - effect of a new villain's weapon. Bruce and Peter work together to not only find a cure for Peter but also to stop the villain. As they do this, they discover new things about each other. Bruce might be inspired by Peter's determination despite being sick, and Peter might learn more about controlling his powers from Bruce.
One possible plotline could be that Peter Parker gets sick with a mysterious illness that even his enhanced healing can't quickly fix. Dr. Bruce Banner, with his scientific expertise, tries to find a cure. Along the way, they bond over their shared experiences of being different and having to deal with the consequences of their powers.
It could be a mad scientist who is jealous of Bruce's work with gamma radiation and wants to create a similar but more dangerous form of it. This new radiation form accidentally makes Peter sick when he gets in the way during one of the villain's experiments.
I really like 'Partners in Science'. In this fanfiction, Peter and Bruce start working together after Peter discovers some strange radiation that he can't figure out on his own. Bruce helps him analyze it, and their friendship grows throughout the story. It has some really cool action scenes where they have to fight off bad guys who are also interested in the radiation.
One common theme is mentorship. Bruce could be a mentor to Peter, guiding him with his scientific knowledge. For example, in some fics, Bruce helps Peter with his web - shooter technology.
One possible plotline could be that Peter Parker and Jill Valentine are brought together by some kind of multiverse event. They then have to team up to fight a common enemy that threatens both their worlds. For example, a new villain who has the ability to spread a virus in Peter's world and summon zombies in Jill's world.
Another theme is the exploration of their dual identities. Peter has to balance his life as a student and Spider - Man, and Bruce has his own struggles with the Hulk. Fanfictions often explore how they can relate to each other's problems and support one another in dealing with their identities.
Perhaps they would team up to fight a common enemy. Peter's agility and combat skills combined with Bruce's Hulk - like strength could make for a formidable duo. They could be facing a villain who is both a scientific threat and a physical menace.
One possible plotline could be that Elsa, with her ice powers, somehow ends up in Peter Parker's world. Maybe she's on the run from some threat in her own realm and Peter, being the friendly neighborhood Spider - Man, decides to help her. They could form a bond as they both have secrets to keep - Elsa with her powers and Peter with his identity as Spider - Man.
In fanfiction, anything can happen. Maybe there was some sort of scientific experiment gone wrong or a strange twist of fate. For example, perhaps Banner's DNA was somehow mixed with that of Peter's parents during a top - secret project. This could lead to Peter being biologically related to Banner in the fanfiction universe.
One plotline could be that they have to protect their city from a new super - villain who is specifically targeting married superhero couples. They'd use their combined powers and their strong bond as a married couple to defeat the villain.
Well, it could also be that in this fanfic, the author is trying to show a different side of Bruce Banner's character development. Maybe in a particular alternate universe or storyline within the fanfic, certain events lead him to this extreme decision. For example, if he loses control of the Hulk and causes a great deal of harm that he can't forgive himself for. It shows how complex his character can be and how far the fanfic writer can take his emotional journey.