How does Harry Potter's learning of dark arts in fanfiction affect the plot?

Yes, it does. Learning the dark arts often leads to unexpected and dangerous situations that can significantly change the course of the story. It can create conflicts, both internal and external for the characters involved.

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.

Dark creatures can be the main source of conflict. For example, if a Dementor plague is introduced in a fanfic, it can drive the characters to find a solution, like a new spell or artifact to repel them. They can also be used to create moral dilemmas. If a character is bitten by a werewolf, how the wizarding community treats that person can be a big part of the plot.

The dark familiar can make Harry more cautious. For example, if it has a tendency to attract dark magic or dangerous situations, Harry has to be more aware of his surroundings. This forces him to grow in his ability to sense trouble.

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.

One potential consequence could be a change in his relationships. His friends might be worried or even turn against him. For example, Hermione, who is always concerned about following the rules and doing what's right, would likely be very troubled. Another consequence could be a shift in his own moral compass. The dark arts are often associated with negative and unethical actions, so he could start to make more morally ambiguous decisions.

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.

The dark twin can act as a source of conflict. It might drive Harry to make different choices, leading to a completely different plot direction. For example, if the dark twin is more power - hungry, Harry might be influenced to seek out darker forms of magic.