One possible plotline could be that Harry Potter decides to take on the Dark Arts in a more proactive way. Instead of just reacting to Voldemort's attacks, he studies the Dark Arts himself to find weaknesses, which is a very bold move for him considering the taboo nature of the Dark Arts in the wizarding world.
Maybe the fanfiction has a plot where Harry Potter stands up to the Ministry of Magic more firmly. He could be leading a movement to change some of the archaic and unjust laws in the wizarding world. This would show his backbone as he goes against the established power structure.
One plotline could be that a young wizard finds the ashes of a phoenix and they start to have strange dreams. The dreams lead him to discover an ancient magic that was thought to be lost. Another could be that a group of Death Eaters try to steal the ashes to use in a ritual to bring back Voldemort in a more powerful form.
One example could be a fanfiction where Harry Potter has an augment that allows him to communicate with magical creatures in a more profound way. This could lead to him forming alliances with creatures like centaurs and house - elves in a different manner than in the original series, and they could help him in his fight against Voldemort.
One interesting aspect could be the character development. Maybe in this fanfiction, Harry Potter shows an even stronger sense of self and determination, which is different from the original. For example, he might stand up to challenges in a more assertive way.
Sure. One example could be a plotline where Harry discovers he's actually an alien from another dimension and his magic comes from that origin rather than his wizarding heritage. Another could be a story where all the Hogwarts houses merge into one super - house and there are no more rivalries or differences between the students. And there might be a plot where the Marauder's Map actually turns into a living being that can guide Harry on adventures that have nothing to do with Hogwarts or the wizarding world as we know it.
One plot could be that the robot is a gift from Dumbledore to Harry. It's designed to protect him from Voldemort in a way that magic can't. So, it has special abilities like detecting dark magic from afar and alerting Harry. Another plot might be that the robot is an experiment gone wrong from a wizard who was trying to combine magic and Muggle technology. It then finds its way to Harry, and he has to figure out how to deal with it and keep it a secret from the Ministry.
One plotline could be that Harry discovers a hidden diary of Kitiara's and it reveals secrets that set him on a quest to protect her legacy.
One plotline could be that Hogwarts is under a new curse that makes the castle itself a dangerous place. The staircases are constantly shifting in a more erratic way, and there are hidden traps everywhere. The characters have to navigate this while also dealing with a new group of dark wizards who have infiltrated the school.
One example could be a fanfiction where Harry discovers his cat familiar has the ability to sense dark magic. The cat might help him in his adventures by warning him of approaching danger.
Some fanfictions that are overly long - winded and full of filler content can be considered the worst. They might have pages and pages of a character just thinking about nothing important or going through mundane daily activities that have no real connection to the main plot of the Harry Potter world.
One example could be a fanfic where Harry has the ability to control time without any limits, which is much more powerful than the time - turning devices in the original series. Another might be a story where Voldemort is made invincible and can defeat any opponent with a single spell, completely overpowering the other characters.