Maybe Harry uses a forbidden spell in the Gryffindor common room and gets caught. As a result, he's grounded and not allowed to participate in the upcoming wizarding event. He then has to find a way to make it up to his housemates.
Dumbledore can also be a significant character. He might be the one who decides Harry's punishment or he could be the one to offer some wisdom to Harry during his time of being grounded. And of course, the teachers like Snape or McGonagall could be involved as they are often the ones who enforce the rules and punishments at Hogwarts.
One possible plotline could be that Harry gets grounded for accidentally using magic outside of school in a Muggle area. This might lead to him having to deal with the disappointment of his friends who can't visit him as easily. Another could be that he gets grounded because he was caught sneaking out to follow a lead on a new dark wizard threat, and the adults in his life want to keep him safe by restricting his movements.
Obviously, Harry Potter himself is the central character. His friends like Ron Weasley and Hermione Granger are also often involved. They might try to visit him while he's grounded or come up with plans to get him out of being grounded. Dumbledore could also play a role, either as the one who enforces the grounding in some cases or as the one who might offer some wisdom to Harry during his time of confinement.
A 'trunks gets grounded fanfiction' might explore various aspects. It could be about Trunks, a character from a particular series, getting punished by being grounded. This could lead to different scenarios such as how he copes with the restrictions, his relationships with other characters during this time, and his internal thoughts and growth. Maybe it shows his struggle between his normal adventurous self and the confined situation he's in now.
One interesting 'Harry Potter Grounded Fanfiction' story could be about Harry being grounded by the Hogwarts professors for sneaking into the Forbidden Forest without permission. He then has to find a way to make it up to them while still dealing with the ever - present threat of Voldemort.
There's also 'Draco's Grounded Dilemma'. In this story, Draco is grounded by Snape for his poor performance in Potions class. It delves into Draco's character, his relationship with Snape, and how he tries to turn his situation around. It's quite interesting as it gives a different perspective on Draco's life at Hogwarts.
A common theme is Arthur learning a lesson. When he gets grounded, it's usually because he did something wrong, like lying or being disobedient. Through the experience of being grounded, he grows as a character.
The most interesting part could be Trunks' attempts to break the rules of his grounding in a clever way. He might use his unique skills or intelligence to find loopholes.
Well, of course, Caillou himself is the central character. His parents also play important roles as they are the ones who ground him and teach him lessons.