Redemption is a common theme. Given the nature of the original story, many fanfics explore how characters continue to seek redemption for their past actions. For example, Shoya's journey of making up for his bullying past.
A fanfic could center around a trip they take together. They could go to a beautiful beach. Shouko would be excited to see the ocean for the first time, and Shouya would be there to make sure she has a great time. They might build sandcastles and collect seashells, and this simple trip could strengthen their relationship even more.
In a fanfic, their relationship could also take a more romantic turn. Ishida, who has come a long way from his bullying days, might start to see Nishimiya in a new light. He could realize his true feelings for her as they spend more time together, perhaps helping each other with their personal growth and insecurities.
A great Silent Voice fanfic idea could be a time - skip story. Imagine what the characters' lives are like years after the events of the movie. Have Shoya and Shoko stayed together? What about their careers? Maybe Shoya has become a sign language teacher, and Shoko is an artist. Or perhaps write a fanfic where a new character enters their lives, someone who doesn't know about their past but slowly uncovers it and helps them heal old wounds in new ways. You could also do a fanfic that's centered around the school events but with a magical twist, like a magical object that helps Shoko communicate more easily and changes the dynamics between the characters.
A fanfic plot could also be about Nishimiya's dreams and how Ishida helps her achieve them. Maybe she wants to be an artist, and Ishida uses his own skills and connections to help her. In the process, they get closer and their relationship blossoms into something more than just friendship.
You can start by looking on fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, including 'Silent Voice'. There are often tags and filters that can help you narrow down your search to find the ones that suit your taste. Another place could be FanFiction.net. It also has a good number of 'Silent Voice' fanfics. Just be sure to read the reviews and summaries to get an idea of what the story is about before you start reading.
In a fanfic, their relationship could be explored in a more magical setting. For example, they find themselves in a world where they can communicate without words, just through emotions. This would enhance their bond as they already have a deep understanding of each other. Shouko's gentle nature and Shouya's determination would shine in this unique world.
It could focus on their daily lives and how they find joy and meaning in the simplest things. Or, it could involve a trip that leads to self-discovery for both. Also, there might be conflicts with other characters that bring out their protective sides for each other.
There could be a new friendship formed between characters that wasn't in the original. Maybe a character overcomes a major fear or achieves a personal goal.