Well, they could be magical symbols. For example, a rune that represents knowledge, like the Fehu rune which is associated with wealth and knowledge in some magical interpretations. It could represent Hermione's thirst for knowledge.
There could be a fanfiction where Hermione gets tattoos during her travels after Hogwarts. She meets Fleur during one of these travels. Fleur, who has always been seen as the more glamorous one, is a bit jealous of Hermione's new edgy look. However, as they get to know each other better, they realize that their differences in appearance are just surface - level, and they become good friends. Hermione's tattoos could be a story of her adventures, and she shares these stories with Fleur.
One interesting story could be where Hermione's tattoos hold magical powers. For example, she has a tattoo that glows when dark magic is near, helping her and her friends detect danger. The piercings could be enchanted too, like a nose piercing that allows her to understand ancient spells when she whispers through it.
Perhaps in the fanfiction, Hermione has a set of tattoos that are in a magical language. Fleur, being from a different magical culture, tries to decipher them. As she does, she and Hermione grow closer, sharing their different perspectives on magic. The tattoos could be a mystery that brings them together, and as they solve it, they form a strong bond.
They might make her more alluring to some characters. For instance, Ron could be initially shocked but then find her new look really attractive. It could add a new layer of tension or excitement to their relationship.
Harry might be a bit surprised at first. But then he'd understand that it's just another way Hermione is expressing herself. He'd probably be more interested in the meaning behind the tattoos.
One idea could be a story where Percy uses his tattoos and piercings to express his rebellious side. Maybe he's in a world where such things are not fully accepted, and he has to fight against the norms.
In some fanfictions, Percy's tattoos could be symbols of his heroic deeds. For example, he might have a tattoo of a trident to represent his connection to the sea. It could be a mark given by a powerful sea deity after a great victory. Another possibility is that his tattoos are a form of map, perhaps leading to a hidden treasure or a secret underwater kingdom that only he can unlock.
His tattoos might make him seem more mysterious to his friends. They could be intrigued by the new addition to his appearance and want to know the stories behind them. This could lead to deeper conversations and strengthen their bonds.
Their tattoos can symbolize loyalty. If they have tattoos that match, it shows that they are always there for each other, just like they were throughout their time at Hogwarts.