In this fanfic, Dumbledore might de - age Harry to give him a second chance at childhood. Harry had a tough childhood, and Dumbledore could think that by de - aging him, he can give Harry a chance to experience a more normal and happy childhood. This would lead to a lot of character development as Harry would have to deal with his past traumas while being in a child's body, and he might also discover new things about his magic along the way.
Well, it could be that Harry discovers some dark secrets about Dumbledore's past actions. Maybe Dumbledore made a deal with some dark forces long ago that Harry thinks endangers the wizarding world. So, in a fit of moral outrage, Harry decides to 'sack' him, which could mean removing him from his position of power and influence at Hogwarts.
Well, perhaps there's a fanfic where Dumbledore travels to the Muggle world for some reason and stumbles upon a baseball game. He gets hold of a baseball bat and realizes it has some interesting properties. Back at Hogwarts, he uses magic to enhance the bat and it becomes a sort of magical weapon that can be used against dark forces. Maybe it has the power to deflect dark spells or even send out bursts of positive energy. And the students are both amused and amazed by this new addition to Dumbledore's arsenal.
One possible reason could be a misunderstanding. Maybe Harry is led to believe that Dumbledore has been hiding something crucial about his parents' death or Voldemort's return. For example, if Harry discovers some encrypted files in Dumbledore's office that seem to suggest Dumbledore had a prior deal with Voldemort, it could lead to a fight. Another aspect could be related to power. If Harry feels that Dumbledore is being too controlling over his actions and the use of his powers, he might lash out. In the wizarding world, power struggles are not uncommon, and Harry, as a powerful wizard in his own right, might want to assert his independence.
One interesting idea could be seeing how the characters' relationships change. For example, if Harry is de - aged, how would his friendship with Ron and Hermione be different? Maybe he'd be more vulnerable and they'd have to take on more of a protective role. Another idea might be the impact on the wizarding world's politics. A de - aged Harry could be a target for those who feared his future power, or he could be used as a tool by different factions. Also, it would be fascinating to see how the de - aging affects his magical abilities. Would he have to relearn spells? Or would his magic be different in some way due to his younger state?
One possible plot could be that Harry discovers a powerful and ancient magic that Dumbledore has been trying to keep hidden. Harry's newfound knowledge and power make Dumbledore fear that he will disrupt the delicate balance in the wizarding world. So, Dumbledore starts to act nervously around Harry and tries to subtly keep him in check.
One possible plot could be that Harry discovers some dark secret about Dumbledore's past actions and decides to oppose him. For example, if he finds out that Dumbledore had some sort of deal with the Dark Arts that he hid from everyone. This would make Harry feel betrayed and drive him to defy Dumbledore.
Well, perhaps Harry discovers some dark secret about Dumbledore that makes him feel he must escape. It could be that Dumbledore is being controlled by some dark magic and Harry realizes he's in danger around him.
It could be that Harry finds out about Dumbledore's secrets regarding the Deathly Hallows. If Harry thinks Dumbledore was more interested in the Hallows for his own purposes rather than for the greater good, it might make him hate Dumbledore.
Harry Potter himself is quite popular. Since he's the main character, seeing him in a de - aged state offers a lot of storytelling possibilities. Hermione Granger is also a common choice. Her intelligence at a younger age could lead to different adventures. And then there's Draco Malfoy. Writers like to explore his character when de - aged as it gives a chance to rewrite his early interactions with the other characters in a new light.
With Hermione and Ron, it would be very different. If Harry is de - aged, they might see him as a younger, more vulnerable friend. Hermione could become even more protective, like an older sister. Ron might be a bit confused at first but would probably still be his loyal friend. Their dynamic would shift from equals to something more like Harry being the 'little one' they need to look after.