The fans themselves could be main characters. They could be shown dealing with their emotions, sharing memories of the singer, and trying to keep the singer's legacy alive. For example, a group of super - fans might form a club dedicated to the singer after the death.
Perhaps in the fanfic, the singer's death is a mystery, and fans start to investigate, uncovering secrets about the singer's life that were previously unknown. This could involve looking into the singer's relationships, unfulfilled dreams, or hidden talents.
The main character could be an ordinary citizen who just happens to be in the wrong place at the wrong time and gets kidnapped. For example, a tourist visiting America.
Obviously, Catherine Rollins herself could be a central figure even though she dies. Her memory could play a big role in the story.
A politician who drives the empire - building policies could be a main character.
A rogue ninja from another village could be the main antagonist. Maybe a ninja from the Hidden Mist Village who has a personal grudge against Konoha and sees Naruto as the symbol of Konoha's strength. So, he devises a complex plan to take down Naruto. This ninja could have some unique and deadly jutsu that Naruto has never faced before, which ultimately leads to his downfall.
It could be a powerful sorcerer who wants to take over Crookshnks' homeland. This sorcerer has dark magic and sees Crookshnks as a threat to his plans, so he devises a scheme to eliminate him.
There could be a young and ambitious trainer named Alex. He's on a quest to become the best Pokemon trainer in America.
Suzaku could be another main character. His conflicted loyalties in the original series could be translated into this new setting. He might be on the side of the British at first, but then have a change of heart after seeing the injustices they are committing in America. His piloting skills could also come in handy, perhaps in battles against the British forces that have advanced mecha technology, similar to the Knightmare Frames in the original 'Code Geass'.
Obviously, America would be the main character. There could also be some sort of selection committee or judges as supporting characters.
Since I don't know the specific fanfic, Sakura is likely one of the main characters as it's in the title. There may also be characters related to her, like her friends or family who are affected by her death in the wave.