He could also affect the plot by influencing Harry's decisions. If the brother is older and wiser, he might convince Harry not to trust certain characters as quickly. For example, he could be suspicious of Snape from the start and keep Harry away from him. This would lead to different interactions between Harry and the other Hogwarts faculty, and potentially change how Harry discovers the truth about his parents and Voldemort. In addition, the brother could play a role in the final battle. Maybe he has a special power that he uses to turn the tide of the battle against Voldemort, saving Harry and many others in the process.
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.
It can create immediate conflict. If a young wizard performs magic accidentally, the Trace can bring the Ministry's attention, like in some fanfics where a minor character's small magic act sets off a chain of events because the Ministry gets involved.
The 'darkness' can make the plot more complex. It might introduce new challenges for the characters that they have to overcome. For example, if there are new dark spells or dark creatures in the fanfiction, the characters need to find ways to deal with them.
Well, it could have a huge impact. If Harry is Bella's brother, it might change the entire dynamic of both the Harry Potter and Twilight universes. For example, Harry's magic could potentially be used to protect Bella from the vampires in Forks. Or perhaps he could use his knowledge of the wizarding world to help her understand some of the strange things happening around her. It could also lead to some interesting crossovers, like having the wizards interact with the werewolves and vampires in a different way than in their original stories.
It can also influence new fan creations. Some new fanfiction writers might be inspired by the more controversial 'Harry Potter Insults Fanfiction' to either create their own version of it or to create works that are a reaction against it. For instance, they might write stories that focus on healing the relationships that are damaged in the insults fanfiction.
The 'harry potter wank fanfiction' can be really divisive in the fandom. On one hand, some people who are into more adult - oriented or edgy interpretations might be drawn to it. But on the other hand, for the majority of fans who love the original values of the series like friendship and magic, it can be off - putting. It can also lead to a bit of a rift in the fandom, with some fans distancing themselves from any association with such inappropriate content.
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.
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 pet might cause trouble for Harry. Imagine it has a mischievous streak and keeps getting into places it shouldn't be, like the Slytherin common room. This would lead to all sorts of awkward and dangerous situations for Harry, forcing him to come up with creative solutions to get out of trouble.
His childish behavior can also add a sense of vulnerability to the plot. Since he's acting like a child, he might not be as capable as he usually is. This could make the threats in the story seem more menacing. For instance, if he's facing a powerful dark wizard but is acting childishly, it heightens the tension because the reader knows he's not in his best form to fight back.
Well, it could change the plot in many ways. For example, he might get early warnings about danger from animals. If a snake is about to attack someone in the Forbidden Forest, the snake could tell Harry, and he could prevent it. This would make him seem even more of a hero and might change the way his friends and enemies view him.