A common plotline is the deepening of relationships. For example, Harry might further develop his friendship with Draco. They could start to understand each other better, realizing that they were both products of their upbringing. Also, Hermione could take on more leadership roles in the school, like starting new study groups or magical research projects, which would be really exciting to read about in fanfiction.
A common plot in eighth year Harry Potter fanfiction is the exploration of friendships. For example, Harry might strengthen his bond with Neville, who has also grown a great deal. There could be a plot where they work together on a new project for Hogwarts, like revamping the Defense Against the Dark Arts curriculum. Also, there are fanfictions where Harry discovers new magical abilities during this year, which leads to a whole new set of adventures.
In some fanfiction, there are plotlines where the Death Eaters have a different strategy. Instead of just following Voldemort blindly, they start to have their own power plays. This could lead to a more complex battle between the good and the bad. Also, there could be a plot where Harry has to deal with a new magical ability that he discovers during year 7, which changes the whole dynamic of the fight against Voldemort.
One interesting plotline could be Harry dealing with the aftermath of the war in Hogwarts. He might be trying to heal the school and its students from the trauma. Another could be his relationship with Ginny evolving further as they face new challenges together in their final year.
One interesting plotline could be exploring more of Snape's inner turmoil during Harry's 6th year. Maybe showing his secret efforts to protect Harry while still maintaining his cover among the Death Eaters.
One interesting plotline could be Harry secretly learning advanced magic on his own to better prepare for the coming battles. He might discover an old, forgotten spellbook in the Room of Requirement.
One interesting plotline could be a deeper exploration of the relationship between Harry and Cedric Diggory before the Triwizard Tournament. Maybe they become unexpected friends and help each other with the tasks in ways not shown in the original book. Another could be a different take on how the other schools' students interact with Hogwarts students. For example, a Durmstrang student might have a secret mission related to the tournament that involves Harry.
One interesting plotline could be an alternate take on Sirius Black's escape. Maybe in the fanfiction, he has some secret magical help that wasn't in the original story. Another could be a story where Harry discovers a new spell during his third year that changes the way he deals with the Dementors. For example, instead of using the Patronus charm right away, he finds a spell that temporarily disables the Dementors' ability to sense fear.
One interesting plotline could be Harry's further exploration of his connection with Voldemort. Maybe he discovers new magical abilities within himself as a result of this connection. Another could be the relationships between the characters evolving in unexpected ways. For example, Neville could take on a more prominent leadership role among the students.
Well, in some 'Harry Potter Year Eight Fanfiction', a plotline might involve the rebuilding of Hogwarts after the war. Harry could play a crucial role in this, using his influence and experience. Also, there could be a story about the integration of Muggle - borns and pure - bloods in a more harmonious way, with Harry leading the charge for equality. There might also be a mystery surrounding some dark magic that still lingers from the war.
One interesting plotline could be Harry's struggle with his new role as a leader in Dumbledore's Army. He has to deal with the pressure of teaching his peers and also keeping the group a secret from Umbridge.