To develop Bill Denbrough in the fanfiction, you could have him learn new skills like self - defense to better stand up to the Bowers Gang. This would not only make him more confident but also show his growth. Regarding the Bowers Gang, explore their relationships with each other outside of bullying. Maybe they have some hobbies in common that they don't show to the outside world. This would add more depth to their characters and make them more than just one - dimensional bullies.
One plot idea could be Bill Denbrough getting caught in a situation where the Bowers Gang tries to bully him into doing their dirty work. Maybe they force him to steal something from the local store. Another could be Bill befriending one member of the Bowers Gang secretly and trying to reform them. And a third idea is that the Bowers Gang discovers Bill's writing talent and tries to use it to their advantage, like writing threatening letters to their enemies but things go awry.
Another way is to join fan groups or communities dedicated to the source material (if there is one). People in these groups often share and recommend good fanfictions. They might even have their own collections or playlists of their favorite Henry Bowers x Bill Denbrough stories.
There are fanfictions that delve into their backstories and how they could have interacted in their childhood in different ways. Maybe a story where Henry Bowers has a moment of kindness towards Bill Denbrough that changes the course of their relationship. These fanfictions often play with the characters' personalities and motives to create unique narratives.
There is a fanfiction called 'Bill Denbrough: The Poet's Soul'. It shows Bill's creative side, how his writing is not just a hobby but a way to deal with his traumas. It has beautiful descriptions of his thought process. And 'Bill Denbrough's Redemption Arc' is also a great read. It's about Bill making amends for past mistakes and growing as a person, which is really inspiring.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions related to Bill Denbrough. Just use the search bar and type in his name. Another good place is FanFiction.net. There are some great stories there too, although you might have to sift through a bit more to find the really good ones.
You could also develop their characters through their relationships with other characters in the fanfiction. If they have friends or foes, how they interact with them can reveal more about who they are. For example, if they have an enemy, do they fight back bravely or try to find a peaceful solution? This can tell us a lot about Reiji and Xiaomu.
Victoria's character can be developed by making her more assertive. In the context of fanfiction with Agent 47, she could start to take on more dangerous tasks and prove herself. For example, instead of always relying on Agent 47 for protection, she could save him in a crucial moment, which would show her growth and independence.
For Hansel, you could show his growth from a scared little boy to a brave young man. Maybe he starts learning self - defense techniques after the encounter with the witch. Gretel could be developed as a more strategic thinker. For example, she could start making maps of the forest and surrounding areas to prevent them from getting lost again.
For Shane, his character can be developed by showing his insecurities behind his confident facade. Maybe he's afraid of not living up to his family's musical expectations. For Mitchie, her growth could be shown as she becomes more assertive in the music world. For example, she could start taking more risks with her music style after being influenced by Shane.
One way to develop both characters is through their relationships with other characters in the fanfiction world. Agent 3 could be made to interact with a mentor - like figure who helps them see a different perspective on their work. Agent 4 could form a friendship with a new recruit and learn to be more of a leader. This would not only develop their individual characters but also show how they influence those around them.