The ending of 'Koe no Katachi' was a bit bittersweet but also provided a sense of closure. The main characters found some sort of resolution and growth.
One interesting concept could be exploring the characters' backstories more in - depth. For example, delving into Shoko's experiences before meeting Ishida in a fanfiction. It can add new layers to her character.
The exploration of different relationships within the 'koe no katachi' universe is also a great concept in fan fiction. This includes not only the main character relationships but also the secondary characters. Writers can create new interactions, friendships or rivalries that expand the world of the story and give readers a fresh perspective on the overall narrative.
First, know the original story well. Understand the personalities of the characters like Ishida's journey from a bully to a repentant person and Shoko's strength in the face of adversity. Then, come up with a unique plot. It could be something as simple as a day in the life of the characters that wasn't shown in the original but still fits the overall tone.
First, re - watch or re - read the original 'koe no katachi' to get a firm grasp of the characters, their personalities, and the overall story arc. This will help you stay true to the essence of the work while creating your own story.
One great 'Koe no Katachi Ishida x Nishimiya fanfic' is 'A New Beginning'. It explores their relationship further after the events of the movie, showing how they grow together. Another interesting one could be 'Our Unspoken Bond' which delves deep into their unspoken feelings for each other.