In some fanfictions, there could be a plot where Harry is forced to learn dark magic to defeat a new, powerful dark wizard. He initially resists but realizes it's the only way. This leads to him having to hide his new knowledge from Dumbledore and others at Hogwarts, creating a lot of tension and secrecy.
Perhaps a plotline where the dark magic Harry learns has unforeseen consequences on his magic. It could start to corrupt his spells in unexpected ways. He then has to find a way to reverse or control these effects while still using the dark magic knowledge he has gained to fight a greater threat.
One possible plotline could be Harry's internal struggle. As he learns dark magic, he has to battle his own moral compass. He might question the boundaries of good and evil. Another could be how his relationships change. His friends might be horrified or try to save him from the dark path.
Another plotline could involve Harry learning ancient magic related to elemental control. For example, he learns to master fire magic in an ancient way that is much more powerful than the regular spells. This newfound power helps him in battles against Voldemort's followers, as he can create massive walls of fire or send out fire - based attacks that are difficult to counter.
In this type of fanfiction, a great plotline could be that Harry stumbles upon a hidden family grimoire. Inside, he finds magic that was thought to be lost. For example, there could be spells for creating protective shields that are stronger than any known spell. Or perhaps he discovers family magic that is related to time - travel, which he then has to learn to control carefully so as not to disrupt the timeline. This discovery of family magic would add a whole new layer to his character development and his role in the wizarding world.
In some 'Harry Learns the Dark Arts' fanfictions, there could be a plotline where Harry uses his newfound knowledge of the Dark Arts to fight against a new, more powerful dark wizard. He might have to walk a fine line between using the Dark Arts in a 'good' way and succumbing to its evil nature. His adventures in this regard could include learning how to counteract dark spells with his own dark - art - based magic, all while trying to keep his soul intact.
One interesting plotline could be Harry discovering a new and ancient form of magic that was lost for centuries. This could lead him on a journey to understand its origin and how to master it.
One interesting plotline could be Harry discovers a hidden potion recipe that gives him the ability to communicate with magical creatures. Another might be Harry has to make a potion to save a sick friend under very difficult circumstances, like with limited ingredients.
Well, there could be a plot where Harry, in his quest to defeat Voldemort once and for all, stumbles upon blood magic as a last resort. He might initially be hesitant but as the situation becomes more dire, he starts to explore its depths. Or perhaps, he is influenced by a new character who claims to be an expert in a different form of magic that includes blood magic, and Harry is drawn into it, believing it could give him an edge in the ongoing battle against the dark forces.
In some fanfictions, Harry might learn family magic that has to do with time manipulation. His family could have had a special talent for seeing into the past or future. As he delves into this magic, he has to learn to control it carefully, as misusing time - related magic can have serious consequences. He may use this new - found magic to prevent certain disasters or to gain knowledge from the past that could change the course of the wizarding world.
Perhaps in the fanfiction, Harry has an encounter with a mysterious magical creature early in his life that imparts special knowledge of magic to him. This early knowledge gives him an edge in his Hogwarts studies and adventures, like being able to solve complex magical puzzles with ease.
In 'Harry Learns Alchemy' fanfic, a cool plotline might be Harry getting tutored by an old and reclusive alchemist who has secrets of the past. This alchemist could teach Harry not just about potions and transmutations but also about the history of magic through the lens of alchemy. Maybe Harry uses his new - found alchemical skills to help Ron, who has been accidentally affected by a faulty spell. He could use alchemy to reverse the spell's effects. Also, there could be a plotline where Harry enters an alchemical competition within the wizarding world and has to outwit his opponents with his unique understanding of alchemy.
One interesting plotline could be Harry's initial struggle with arithmancy concepts. He might find it difficult to understand the complex numerical magic at first, but as the story progresses, he starts to see how it relates to his overall magical abilities.