Well, it could be about Harry using his magic in a new and creative way to build a castle. Maybe he's tired of Hogwarts and wants to create his own place. He might use spells like Wingardium Leviosa to lift the stones and Protego to protect the construction site.
In this fanfiction, Harry could be building a castle as a part of a school project at Hogwarts. He teams up with his friends Hermione and Ron. Hermione uses her extensive knowledge of magic theory to plan the structure, Ron helps with the heavy lifting using magic, and Harry, with his natural leadership and charm, coordinates the whole process. They might face a time limit and competition from other students, which adds more excitement to the story.
Hermione could be one of the main characters. She would be very helpful as she is smart and well - read in magic. She could offer advice on the best spells to use for building the castle and how to make it structurally sound.
The castle might be a grand and ancient structure. It could have high turrets reaching towards the sky, with thick stone walls that have withstood the test of time. There might be a large moat around it, filled with mysterious creatures. The interior could be filled with secret passages and rooms, just like Hogwarts. There could be a great hall with a long table for feasts and a fireplace that always has a warm fire burning.
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.