Well, perhaps the child could be in danger from some sort of scientific experiment gone wrong. Bruce, using his Hulk powers, could smash through barriers to reach the child. He might have to fight off some other mutated creatures along the way.
His dual identity is crucial. As Bruce Banner, he can think rationally and come up with a rescue plan. For example, he can analyze the situation, find the safest route to the child. But as the Hulk, he has the brute strength to overcome any physical obstacles. If there are huge boulders in the way of reaching the child, the Hulk can easily move them.
Bruce Banner fanfic, when not involving inappropriate content, usually has a strong focus on his internal conflict. He is a man of science who has been burdened with this uncontrollable alter - ego. Fanfics can show his attempts to find a cure or at least better manage the Hulk. In addition, his relationships within the Avengers can be a central aspect, whether it's his mentorship of Peter Parker or his camaraderie with Steve Rogers.
One key element is the contrast between their characters. Bruce is often seen as a meek and brilliant scientist, while Loki is a cunning and powerful god. Their interactions based on this contrast can create interesting storylines.
First, you need to decide on the basic plot. Will it be a story where they meet in one of their worlds, or will it be a new world entirely? Then, think about how their personalities will interact. Bruce is a brilliant scientist with a temper problem, while Harry is a brave and determined wizard. You can create interesting scenes based on their different skills and mindsets. For instance, Bruce could use his scientific knowledge to analyze a magical phenomenon, and Harry could use magic to protect Bruce from danger.
One idea could be a story where Bruce and Natasha are on a mission together in a remote location. They have to rely on each other's skills to survive. Bruce uses his scientific knowledge to figure out a way to stop a dangerous experiment gone wrong while Natasha uses her combat skills to protect them from the threats around. Their relationship gradually deepens as they face challenges side by side.
Another fanfic concept could involve Harry Potter somehow ending up in Bruce Banner's world. He might be there to learn about modern Muggle technology and end up meeting Bruce. Together, they could face a threat that requires both magic and scientific knowledge to defeat. For example, a villain who is using both dark magic and advanced technology to cause chaos.
In a 'bruce banner death fanfic', it could be that Banner is tricked by a powerful villain into believing that his continued existence is a threat to all of humanity. Depressed and believing he has no other option, he takes his own life. This could lead to a complex exploration of the ethics of superheroes and the power they hold, as well as the impact of Banner's death on his friends and the world at large.
Bruce Banner is still alive in the comics. His storylines have many ups and downs, but death hasn't been the ultimate fate for him. He keeps coming back stronger.