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.
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.
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.
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.
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.
In a fanfiction, Harry Potter could be in a situation where he needs to recruit players with strong muscles for the Quidditch team. He might search through Hogwarts to find students who have been secretly training and have the physical strength required for the tough positions in the game, like the Beaters.
Ron could also be a great ally. His loyalty and courage would be important. He might be in charge of security, using his skills in wizard chess to plan out defensive strategies. And of course, the Weasley family in general could offer a lot of support, with their various skills and connections in the wizarding world.
Maybe an old wizard who has been secretly studying the combination of different magical universes for years. This wizard could have made some initial attempts at building a lightsaber - like weapon in the past and can share his experiences and mistakes with Harry. He would teach Harry about the balance between the light and dark side of this new form of magic, just like the Jedi teachings in Star Wars.