Banana Fish is not based on real events. It's a fictional narrative that takes the reader/viewer into a made-up world with its own unique plot and characters.
One popular theme is exploring the relationship between Ash and Eiji in more depth. Another is creating alternate endings for the story. Also, some fanfictions focus on the backstories of the other characters in the Banana Fish universe.
A common theme is the alteration of the original story's tragic elements. Banana Fish has a lot of sad and tragic parts. In time travel fanfictions, authors often use the time travel concept to make things better. This could be saving a character from death, or preventing a particular betrayal. It gives the fans a sense of hope and an alternate view of the story that they love.
One common theme is friendship. Their bond is often explored in different ways, like how they support each other through tough times. Another theme is sacrifice. Ash, in particular, is often portrayed as being willing to sacrifice a great deal for Eiji.
Banana Fish is purely fictional. The plot, characters, and events were all imagined by the writer to create an engaging and unique narrative. There's no real-life inspiration behind it.
In Banana Fish fanfiction, Ash is a top choice for writers. His troubled life, from his experiences in the gang world to his inner turmoil, gives so much material. But don't forget about Shorter. His loyalty and his connection to Ash make him an interesting character to write about. He has his own story arc in the original that can be expanded in fanfiction. And then there's Eiji, who is sort of an outsider coming into this dark world, and his interactions with the other characters can be really interesting to explore.