It could change the power dynamics. If Harry finds a loophole in a powerful spell or a rule, he might become more of a threat to the Dark forces. For example, if he can use a spell in a new way due to the loophole, he can better defend against Voldemort's attacks.
His discovery of the loophole might lead to new friendships or alliances. Maybe other characters are interested in this loophole too, and they team up with Harry. For instance, some students who were previously on the fence about helping Harry might see the potential of the loophole and decide to join him in his fight against the bad guys. Also, it could completely change the course of a battle. If the loophole is related to a battle spell, Harry could turn the tide of a losing battle and save many lives.
The discovery might attract the attention of villains. They would want to either capture the baby phoenix for its powers or stop Harry from using it to his advantage. This would lead to more battles and challenges for Harry and his friends.
The discovery could create a new arc in the plot where Harry has to learn to master the power of Merlin's wand. It might be difficult as the wand could have its own will.
If Harry Potter found the Necronomicon in fanfiction, it could have a huge impact on the wizarding world. It might open up new areas of magic study that were previously unthinkable. But it also poses a great threat. Dark wizards could be attracted to the power it represents and start causing trouble. The Ministry of Magic would likely get involved, either to protect Harry or to confiscate the book.
It can make the plot more focused on recovery and justice. The story could revolve around Harry getting the help he needs and the search for the abuser if not already known.
Her discovery can bring new plotlines. Maybe Mary tries to keep Jess safe from the supernatural threats she knows about. This could involve her teaching Jess some self - defense or warning her about certain monsters. It also might make other characters reevaluate their relationships with Dean, as Mary's opinion often matters in the Supernatural world of the fanfiction.
His refusal to talk can also lead to misinterpretations in the fanfiction. Other characters might misjudge his motives or feelings because he's not communicating verbally. This can then cause conflicts or plot twists. For example, Ron might think Harry is angry with him when in fact Harry is just dealing with his own issues privately.
Let's say his pet is a dementor. It would change the whole atmosphere of the story. The characters would constantly be in fear of having their souls sucked out. It could also be used to guard important places. For instance, Voldemort might place it near the entrance of his lair. This would make it extremely difficult for the heroes to approach as they have to deal with this terrifying creature first. Also, it could spread a sense of despair among the wizarding community, making people more vulnerable to Voldemort's control.
His anger could drive him to take risks he wouldn't normally take. Maybe he decides to go after a powerful enemy on his own without proper backup because he's so angry that he doesn't think clearly. This could put him in great danger and also change the course of the story as the other characters then have to scramble to save him.
The protective brother can change the plot significantly. For instance, if he's highly skilled in magic, he can prevent Harry from getting into some of the dangerous situations he faces in the original series. He might be able to stop Harry from entering the Triwizard Tournament by interfering with the Goblet of Fire's magic. This would then change the entire course of that year at Hogwarts and Harry's relationship with Cedric Diggory and others.
The pet bat could make Harry's life more interesting. It could be his companion during lonely times, like when he's in his dormitory and the others are asleep.