In a 'Harry Potter Dies' fanfiction, the plot often takes a dark turn. The hope that Harry represented is gone. This could lead to the Ministry of Magic falling apart, as they had put so much faith in Harry. Hogwarts might also become a more dangerous place as the students lose their sense of security. The Death Eaters could become bolder, and the remaining good characters have to scramble to hold things together. It can also lead to new alliances being formed as characters who were once on the sidelines are now forced into the fray.
Often, it changes the original plot by allowing Harry to prevent certain deaths. For example, he might go back in time to save Sirius Black or Dumbledore. This alters the emotional landscape of the story significantly.
The fanfiction might also change the original plot by introducing family - centered subplots. Since Harry has a sister, there could be more focus on the Potter family history and legacy. Maybe the sister discovers something about their parents that Harry didn't know before. This can lead to a whole new set of quests and revelations, as they try to understand their family's past better. Also, the sister's existence can influence the way Voldemort views the Potters. He might see her as another threat or as a means to get to Harry, which changes the nature of their conflict.
It can make the readers more aware of the vulnerability of Harry. In the original story, he overcomes many challenges, but in these fanfictions, the added abuse aspect shows how much more difficult his life could have been.
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.
In the original story, Harry survives his time at the Dursleys and goes on to Hogwarts. But in this fanfiction, his death at the Dursleys' is a huge deviation. It means there's no journey to Hogwarts, no making friends with Ron and Hermione, and no learning magic in the traditional way. It's a completely different path from the start.
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.
His friends like Mary Jane would be devastated. She might lose her will to carry on at first, feeling like a part of her has died with Spider Man.
Well, a 'Harry raised by Andromeda' fanfic can completely transform the original story's plot. Andromeda, being a Black, has her own connections and knowledge of the wizarding world. She can introduce Harry to different aspects of magic and magical families that he might not have known otherwise. His growth as a wizard is different. He may not rely as much on his friends for emotional support as he gets it all at home. This can change the dynamics of his friendships at Hogwarts. Also, in the battle against Voldemort, Harry may have different resources and allies due to Andromeda's influence.
The plot might develop by showing how the world changes without Tails. Maybe his inventions were crucial for the protection of their world, and now they have to find new ways to survive. It could also involve other characters trying to bring Tails back through some magical or technological means.