Well, it could be that Harry Potter steals a ship in a moment of desperation. The wizarding world is in chaos, and he believes that there is something crucial out at sea that can turn the tides. He takes the ship without permission, perhaps from the Hogwarts docks. Once on the ship, he has to figure out how to navigate it using a combination of magic and his own wits. There could be sea monsters and other magical creatures that he encounters during his voyage, and he has to use his spells to either avoid or defeat them. He may also find that the ship has some hidden secrets or powers of its own that he gradually discovers as he sails further into the unknown.
Maybe Harry Potter steals a ship to escape from some dark wizards who are chasing him. He uses his magic skills to sail the ship stealthily and heads towards a hidden island where he hopes to find a powerful magical artifact to defeat the dark forces.
Ron and Hermione are the most likely ones. They are his best friends and have always been by his side through thick and thin. They would probably follow him without hesitation and use their own skills, like Hermione's vast knowledge and Ron's loyalty and practical skills, to assist in the ship - stealing adventure.
One possibility is that Harry is escaping from some danger in the wizarding world. He boards the ship to get as far away as possible. While on the ship, he discovers that the ship has its own secrets and magic. He gets involved in the ship's internal affairs, like helping the crew deal with a curse that has been plaguing them.
Maybe Harry steals wands to gain more power in the wizarding world. He could be on a secret mission to collect powerful wands that are hidden away. By stealing these wands, he might be trying to tip the balance of power in his favor against some dark forces.
The plot could involve Harry building the ship with the help of his friends. They could all contribute their skills. For example, Hermione could use her vast knowledge of magic theory to ensure the ship's construction is perfect, while Ron could offer his practical skills. They might be building the ship to escape from some new dark force that has emerged in the wizarding world. Harry's ship would then be their means of getting away and finding a new place to start over. It could also be a ship that has special magical properties, like being able to fly or travel through time.
Well, it could be that Dumbledore steals Harry's Invisibility Cloak for some 'greater good' he believes in. Maybe he thinks it's too dangerous for Harry to have at a certain point in time.
Well, perhaps Harry Potter uses his magic to gather materials from all over the wizarding world. He might use spells like 'accio' to bring in rare woods and magical metals. Then, with the help of his friends like Hermione, who could research the best construction spells, and Ron, who provides the brute force when needed, they start building the city - ship. It could be a floating city with different areas for Hogwarts - like classrooms, places for Quidditch, and areas for magical creatures.
One possible plot could be a miscommunication spell gone wrong. Harry Potter accidentally gets caught in a spell that makes other people think he's trying to'steal' their wives. In reality, he's just trying to fix the spell and return things to normal. Along the way, he has to deal with the misunderstandings of his friends and the magical community, all while trying to figure out how to reverse the spell.
He might go on the run and form an alliance with some unlikely characters. For example, he could team up with some dark wizards who also have a grudge against Gringotts. But this would be a very risky move for him.
The fanfic could center around Harry's life outside of the main events of the books. For example, it could explore his summer holidays in more detail. He could be spending time in the Muggle world and using his magic in creative ways to solve normal, everyday problems. He might use magic to help his neighbor fix a broken fence without the neighbor realizing it was magic. This would add a new dimension to his character and show how he can adapt his magical abilities to different situations.
One possible plot could be that Harry Potter is hired by Sherlock Holmes to help with a case that involves dark magic. Harry uses his knowledge of the wizarding world to uncover clues that Sherlock couldn't find with just his deductive skills.