Some of the Hogwarts professors could also assist. Professor Flitwick could use his charms expertise to add special touches to the city - ship, like making the doors open with a unique charm. Professor McGonagall could use her transfiguration skills to transform materials into the needed shapes for the construction. And Neville could also play a role. He has grown into a powerful wizard, and his knowledge of herbology could be used to create a beautiful and functional garden on the city - ship.
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.
Dumbledore might help in a spiritual or advisory way. Even though he's passed away, his wisdom could be passed on through memories or some magical means. Also, Hagrid could be a great help. He could assist in building areas suitable for magical creatures with his love and understanding of them.
Hermione would be a great help. Her vast knowledge of magic and books could provide the necessary information for building the planet, like the right spells and magical theories.
There could be lost wizards who got stranded on the ship for some reason. Maybe they were on an expedition and something went wrong.
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.
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.
One interesting Harry Potter city - ship fanfiction could be about Hogwarts as a floating city. Imagine the castle and all its surrounding areas floating in the sky, with magical bridges connecting different parts of the city. Another could be a fanfic where Diagon Alley expands into a full - fledged city, with new and unique magical shops and homes for the witches and wizards.
His character could develop in terms of leadership. As he is building the ship, he has to take charge and make decisions. This would make him more confident in his leadership abilities.
The Ministry of Magic would probably oppose it. They are always concerned about maintaining the secrecy of the wizarding world and a stargate could potentially expose it to non - magical beings or lead to unwanted magical interference from other realms.
He could have adventures like exploring uncharted magical islands. Maybe he'd encounter new magical creatures and have to use his wizarding skills to either befriend or fend them off. For example, he might come across a group of mermaids who guard a powerful magical artifact on an island.