Another reason could be his search for a cure. Bruce may leave the Avengers in fanfic to focus solely on finding a way to permanently separate himself from the Hulk. He might feel that the team environment is not conducive to his research and that he needs to be on his own to make progress.
One common theme is the exploration of their unspoken attraction. They both are complex characters and fanfics often play with how they might come to terms with their feelings for each other.
One possible reason could be a difference in ideology. Maybe Spider - Man has a more local - friendly approach to crime - fighting while the Avengers are often involved in large - scale, global threats. He might feel that his skills are better utilized in his own neighborhood, like Queens, rather than in the grand, often cosmos - level battles of the Avengers.
One common theme is the idea of finding solace in each other. Both Bruce and Natasha have troubled pasts, and in fanfictions, they often find comfort in their relationship. For example, Bruce can understand Natasha's feeling of being an outsider sometimes because he also feels like he doesn't quite fit in due to the Hulk.
One reason could be to seek a new life away from the political drama in Konoha. Naruto has always been in the spotlight and has faced many political challenges. Hinata, being by his side, might support his decision to leave for a quieter place. Maybe they want to raise their family in a more peaceful environment, away from the constant threat of enemies targeting Konoha.
One reason could be that Bruce is being too reckless in the fanfic. Alfred has always been there to keep Bruce safe and if Bruce is constantly putting himself in unnecessary danger as Batman, Alfred would be disappointed. For instance, if he jumps into a fight without proper preparation or a well - thought - out strategy.
Well, Bruce could leave in fanfic for the sake of self - discovery. He's constantly struggling with his identity as Bruce and as the Hulk. Fanfic authors might write him leaving to find a place where he can better understand himself without the distractions and pressures of the Avengers team.
One common theme is protection. Tony often takes on the role of protecting Bruce in these fanfictions, especially when Bruce is in his ageplay state. Another theme could be discovery, as Tony discovers new aspects of Bruce's personality through this ageplay situation.
One common theme is his struggle with the Hulk. Writers often explore how he copes with this dual identity, like in stories where he tries to find a permanent cure. Another theme is his relationships, especially his friendship with the other Avengers. For example, how Tony Stark might help him with his scientific research related to the Hulk.
It could be to introduce unexpected plot twists and keep readers on their toes. Maybe a character has to leave for personal growth or to create tension.
Characterization. If it really nails Bruce's complex personality, it stands out. For example, showing his intelligence as a scientist and his insecurities about the Hulk.