The fanfiction might tell the story of young Harry being shrunk and ending up in a toy house. In the toy house, he has to adapt to his new small size. He could encounter various challenges such as dealing with a bully toy or trying to find a way to return to his normal size. He might also learn important lessons about friendship and courage through his interactions with the toys.
There could be a wise old teddy bear that acts as a mentor to Harry. It might have been in the toy house for a long time and knows all its secrets.
In this fanfiction, the toy house could be a gift from Dumbledore. It has the power to shrink or grow depending on Harry's needs. Inside, the toys are actually the souls of great wizards who have been trapped. Harry has to free them by passing a series of magical tests, and in return, they teach him advanced magic that no one else at Hogwarts knows.
Maybe Harry decides to build a house in a magical forest. He uses his magic to gather the materials like levitating the logs. He might also face some challenges like protecting the construction site from magical creatures that want to play with the building supplies.
She could be making toys that are actually enchanted with real spells. For example, a toy dragon that breathes out harmless but cool - looking 'fire' which is actually a spell. And the story could revolve around her trying to keep these toys from falling into the wrong hands, like some dark wizards who might want to use the spells for evil.
It could be a dark wizard who wants to take over the toy house for its magical properties. Maybe he believes that there are powerful artifacts hidden within the toys that can make him invincible.
Maybe Harry builds the house as a sanctuary for his friends who lost their homes during the war. He'd use his knowledge of magic and the help of Hagrid, who would supply some magical building materials. The house could have rooms that change according to the mood of the occupants, just like the rooms in Hogwarts.
Perhaps the toy is a little magical animal figurine that can move and talk, and it's designed to keep Hermione company during her long hours in the library. Harry might have made it as a way to make her smile during her stressful study times. He could have spent a lot of time researching spells and materials to create something truly special for her.
Maybe the house elves train Harry in the art of stealth. Since they are so good at moving around unnoticed in the wizarding world. They could pass on those skills to Harry, which would help him greatly in his battles against Voldemort and his Death Eaters. Harry could then use this new - found stealth to gather intelligence or surprise his enemies.
Well, perhaps in this fanfic, the fifth house could be a house for those with mixed magical abilities. Harry might be sorted into it because he has both Gryffindor - like courage and Slytherin - like cunning. He could then meet new friends who also don't quite fit into the traditional four houses. They would go on adventures together, facing challenges that require the unique combination of traits that the fifth house represents.
Perhaps Voldemort has cast a powerful spell on Harry that the normal wizarding community can't break. But the house elves, with their different kind of magic, are not affected in the same way. They detect Harry's plight and use their loyalty and cleverness. For example, they might create a magical shield around Harry that slowly weakens the dark spell, eventually saving him.