In such a fanfic, perhaps Harry might rebel because he feels the Order is being too overprotective. He could think they are holding him back from fully exploring his power to defeat Voldemort. For example, they might not let him go on certain dangerous missions he believes are crucial.
One possible plot could be that Harry rebels due to differences in ideology. The Order might be more focused on traditional ways of fighting dark magic, while Harry has new and radical ideas. He could rebel by trying out new spells or magical techniques that the Order frowns upon. He might believe that these new methods are the key to victory over Voldemort, but the Order sees them as too risky.
The plot could involve Harry getting tired of the Order's strict rules. He may start to question their strategies. He might find out some secrets within the Order that make him lose trust in them. Then he decides to go his own way, gather his own allies like some outcast wizards who also don't agree with the Order, and form a new group to fight against Voldemort in his own style.
Hermione could be one of his allies. Despite her usually rule - following nature, she might side with Harry if she sees the merit in his rebellion. Also, Ron would likely be there, as he has always been by Harry's side through thick and thin.
Well, it could be about a group of regular Pokémon who are tired of being overshadowed by the legendary Pokémon. Maybe they start to form an alliance to demand equal treatment, like access to better habitats or more respect from trainers. They might use their unique skills in ways that the legendaries haven't expected, creating a whole new dynamic in the Pokémon world.
One possible reason could be a difference in ideology. Maybe Harry starts to question Dumbledore's decisions regarding the war against Voldemort. For example, Dumbledore's secrecy about certain things might make Harry feel left out and manipulated. Another reason could be personal pride. Harry has grown into a powerful wizard and may not want to be under Dumbledore's constant guidance and control any longer. He might think he can handle things on his own better. Also, it could be due to the influence of other characters. If some new friends or allies have different views on Dumbledore, they could sway Harry's opinion.
It could be Pikachu. Pikachu is a very popular Pokémon and it's not a legendary. It might get tired of seeing the legendaries always in the spotlight and decide to lead the rebellion. Another possibility is Charizard. Charizard is strong and has a bit of a rebellious streak already, so it would make sense for it to be involved in something like this.
The change in the storyline would be multi - faceted. Emotionally, Harry would be on a different journey. He would be dealing with feelings of betrayal (if he believes Dumbledore has wronged him) and determination to prove himself right. In terms of the plot, the search for Horcruxes might take a different turn. Harry might not follow Dumbledore's clues as closely. He could end up finding Horcruxes in different ways or even missing some if he doesn't have Dumbledore's wisdom. And the final confrontation with Voldemort would be more unpredictable. Harry's rebellion could lead to a more evenly matched battle as he might not have all the advantages that Dumbledore's guidance would have provided.
The main plot of such a fanfic could be something like a time travel element where Harry has to correct a past mistake to ensure a better future. Or it could be a story about a new magical government order that changes everything for wizards and witches.
Well, perhaps in the 'harry potter in the order fanfic', a major character's magical ability is suddenly revealed to be much different than what was originally thought. Like Hermione, who was always known for her book smarts, might have a hidden and extremely powerful combat - based magic that gets uncovered during a crucial battle. This would completely change the dynamics of how she is perceived within the Order and how she can contribute to their cause.
It could be that the Order has a plan that only Harry can be a part of. His unique experiences and skills, like his ability to resist the Imperius Curse and his connection to Voldemort, are valuable. So they summon him to put these to use in their fight against the Dark side.
Well, it could be about Harry unknowingly crossing over with a character from another magical universe. Maybe he meets a young wizard from a different school who has a unique power that could help in the fight against Voldemort. They could team up secretly and use their combined magic to outwit the Death Eaters in ways the Order of the Phoenix hasn't thought of yet.
One interesting plot idea could be exploring the backstories of the minor characters in the Order more deeply. For example, what made them join the Order and their personal struggles during the fight against Voldemort.