Perhaps it's because he brought home some dangerous magical creatures without permission. His family might be in chaos without his guidance while he's grounded. For example, the twins could be getting into even more trouble without his fatherly watch.
Another possibility is that he got into an argument with a higher - up at the Ministry over some new regulations. His strong sense of justice made him oppose the rules, and as a result, he was grounded, which then impacts his relationship with his family as they try to deal with the consequences.
His family might be shocked at first. Ginny, for instance, would be really confused as Arthur is usually the one keeping order. She might try to be extra good to make up for his mistake.
One possible plot could be that Rose Weasley is doing some experimental magic in the Weasley family home. Arthur Weasley, being the curious and experienced wizard he is, comes across a strange lemon - like object that Rose has. It turns out this 'lemon' is actually a magical artifact from another time. They then embark on a journey to figure out its origins and how to use it safely.
One possible plot could be that Arthur Weasley accidentally uses some experimental magic that causes an unexpected interaction with Hermione. For example, his fingers might emit a strange energy that transfers some of his knowledge to Hermione in an unorthodox way, leading to a series of adventures where they have to figure out how to reverse or control this new connection.
One possible plot could be the OC, now as Arthur Weasley, trying to modernize the wizarding world's technology in a more radical way. Since the OC has different perspectives from the original Arthur, they might introduce new ideas like a more efficient postal system using magic and Muggle technology combined. This could lead to some resistance from traditional wizards but also gain support from the younger generation who are more open to change.
It might focus on the character's struggle to fit in and overcome the stereotypes. Maybe they have to prove themselves differently than their family's Gryffindor reputation.
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.
One plot idea could be that Arthur discovers Ginny has a special magical talent that she's been hiding out of fear. He then helps her gain confidence in using it. For example, it could be a unique form of wandless magic. Another idea is that they go on a magical adventure together, like exploring an ancient, forgotten wizarding site. There, they encounter strange magical creatures and have to use their combined knowledge and skills to get out safely.
Perhaps Seto got into a fight with a powerful enemy and in the process, he put others in danger. His guardians or the people in charge decide to ground him to teach him a lesson about responsibility. During his grounding, he could start to reflect on his actions and come up with new strategies to face his enemies. He might also discover new things about himself while being alone in his grounded state.
Maybe Harry accidentally frames Weasley for something. It could be due to a misunderstanding, like a spell gone wrong that makes it seem like Weasley was involved in some illegal magic use.
Well, it could start with an old, long - forgotten grudge between the two families. Maybe a misunderstanding in the past regarding magic or a family heirloom. For example, a powerful magical item that was thought to be stolen by a Potter from a Weasley ancestor, which led to the start of the feud.