The Death Eaters could be the antagonists. Even though Voldemort is gone, some of his followers might not want Harry to succeed in building a starship as they fear it could bring more power to Harry and his friends, and they still hold grudges against him.
The plot could involve Harry using his knowledge of spells and potions to create a unique starship. He could also recruit his wizard friends to help him. For example, Hermione could use her vast knowledge to research the best magical elements to incorporate into the ship's design, while Ron could provide his loyalty and physical strength for the more labor - intensive parts of the construction. And they might face challenges from the wizarding world, like the Ministry of Magic trying to stop them for fear of revealing magic to the Muggle world through their starship adventure.
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.
Well, in some Harry Potter starship fanfiction, there might be the idea of using Patronuses as a form of energy source for the starship. Since Patronuses are powerful positive magical manifestations, they could potentially power up the ship's engines. Also, there could be a story where the starship has to travel through different magical dimensions, like the ones that might be related to the Forbidden Forest's hidden areas, and the crew has to use their knowledge of Hogwarts magic to survive.
Harry Potter himself is often a popular choice. Fans like to imagine him as the captain or a key member of the starship crew, using his Gryffindor courage to face various space - related challenges. Hermione Granger is also popular as she can bring her vast knowledge of magic and her quick - witted problem - solving skills to the crew.
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.