He could be a target of a dark wizard who wants to steal his new - found power associated with the white hair. In this fanfiction, Harry has to constantly be on the run and use his wits and magic to stay alive. The white hair might be a sign of a connection to an ancient magic that the dark wizard desires.
One idea could be that Felicity is kidnapped by a rival of her friend. Her hair in a bun might be a key detail, like the kidnapper uses it to hold her captive in some way, perhaps by tying her bun to a post. Then her friends have to figure out where she is based on the few clues she manages to leave behind, which are related to her bun.
One idea could be that Saitama's hair gives him some new, unexpected power. For example, his hair could start to glow and act as a power amplifier, allowing him to defeat enemies even more easily than before. This would be a fun twist on his already overpowered character.
One idea could be that the admiral with black hair and glasses is leading a space exploration mission to a distant and mysterious planet. The crew faces various challenges like strange gravitational fields and alien life forms, and the admiral's intelligence and leadership, symbolized by his glasses perhaps, help them through.
Another concept is a historical fanfiction set in an ancient Japanese - inspired world. The character with pink hair as sakura and green eyes is a princess. She is betrothed to a prince from a neighboring land, but she has her own dreams and desires. The fanfiction could explore her struggle between her duty and her personal wishes, and how her appearance plays a role in the political and social situations of the time.
Perhaps the black hair tie is a reminder of a past love. Felicity is constantly haunted by the memories associated with it. She meets someone new in the fanfiction, and the presence of the hair tie creates a conflict as she tries to move on from her past while still holding onto this memento.
A fanfiction could explore a scenario where Harry Potter's long black hair is enchanted to hold spells. So he has to learn how to use his hair as a magical tool. For instance, he could cast spells by twirling a strand of his long hair.
Maybe she's a competitive swimmer. The hair tie on her wrist is from her best friend who couldn't make it to the big championship. Every time she feels tired during the race, she looks at the hair tie and remembers her friend's support, which gives her the extra boost to keep going and win the race. In the fanfiction, you could explore her friendship and her journey in the world of competitive swimming.
One idea could be that the girl is at the bowling alley waiting for her turn when she notices a guy staring at the hair tie on her wrist. It could be a memento from someone special, like her late grandmother who used to take her bowling. This could lead to a heartwarming conversation between the girl and the guy as she shares her story.
One idea could be a fanfiction where Saitama's hair suddenly gains sentience. It starts to give Saitama advice on how to be more heroic in a sassy way. For example, it might tell him to change his plain hero costume to something more flashy to attract more fans.