Another interesting plot could be Nagini's role in the wizarding world if Voldemort had never found her. She was a powerful and intelligent magical being. Maybe she would have become an important figure in the wizarding community on her own, working towards the betterment of magical creatures or something similar. This would show her potential outside of being Voldemort's servant.
Some fanfics might explore Nagini's connection to the other Horcruxes. Since she was the last Horcrux to be destroyed, perhaps there was some kind of communication or power exchange between her and the other Horcruxes that we didn't see in the books. This could add a whole new layer of mystery to the story.
A really cool plot in some fanfictions is when Nagini has a secret power that even Voldemort doesn't know about. This power could be something that has the potential to change the outcome of the wizarding war. Maybe it's a power that can only be unlocked by a certain person, like Harry, and this creates a whole new set of adventures and challenges.
A plot that I find very intriguing is when a character travels back in time to prevent the death of a loved one. For example, in some fanfics, Harry travels back to save Sirius. This creates a lot of tension as he has to deal with the time - turner rules and the possible consequences of changing the past.
The plot where Murtagh teaches Harry some of his unique magic techniques is really cool. It shows how different magical systems can interact. For example, in a fanfic I read, Murtagh showed Harry how to use a spell that was common in his world but unheard of in Hogwarts.
One of the most interesting plots could be Snape having to protect Harry in a very unexpected way. For example, in a fanfic where dark magic has been unleashed in a different form and it targets Harry specifically. Snape, despite his initial reluctance, realizes that Harry is the key to stopping it and has to put aside their differences. Another plot could involve Snape and Harry being transported to an alternate magical realm where they have to rely on each other to survive. They slowly start to understand each other's past traumas and develop a grudging respect. A third plot might be that Snape discovers a long - lost prophecy about Harry that changes his perception of him completely and he becomes his secret mentor in a much more involved way than in the original books.
In some fanfics, the plot might involve Harry using his snake form to spy on the Death Eaters. For example, he could slither into their secret meetings undetected. Another interesting plot could be that his snake form is somehow connected to Salazar Slytherin. Maybe he discovers that his ability to turn into a snake is part of an ancient bloodline magic.
A common plot in 'Harry Potter Dolores Umbridge Fanfiction' is her getting a taste of her own medicine. For example, in some stories, she is put through the same strict and cruel rules she imposes on others. It's a satisfying plot for those who dislike her character in the original books.
There are also fanfictions that look at Helena Ravenclaw from the perspective of her motherly love for Hogwarts. She's like a protective spirit in these stories, and the plot might involve her guiding certain characters, like Harry or Hermione, through difficult times at Hogwarts. It's a heartwarming and different take on her character that shows her connection to the school in a more emotional way.
Well, in some fanfics, like 'The Darker Side of Redemption', Lucius is given a chance at redemption after the fall of Voldemort. Harry plays a key role in this process, either by being a reluctant helper or an unexpected friend. The plot often involves exploring the darker parts of the wizarding world that were not fully shown in the original books.
One interesting plot could be Harry using his magical skills in veterinary work. For example, he might use spells to heal injured animals quickly. Maybe he'd have to deal with magical creatures that are acting strange and he has to figure out if it's a magical illness or something else.
Well, there's a plot where Penelope is secretly a powerful witch with ancient magic in her bloodline. In this fanfic, she has to hide her true power from everyone at Hogwarts. It makes for a really exciting read as she tries to balance her normal school life with the secrets she's keeping.