Another plot idea is if Hermione turns into a centaur. In this form, she would gain incredible strength and knowledge of the Forbidden Forest. She could then use her new - found abilities to help Hagrid protect the forest and its inhabitants. She might also have to deal with the other centaurs' initial distrust of her, as she was once a human.
One idea could be that Hermione turns into a phoenix. She could then use her new form to fly around Hogwarts, exploring places that are usually off - limits. She might also discover she has the power to heal with her tears, like a phoenix does, and help injured students or magical creatures on the school grounds.
It could be a group of pure - blood supremacists. They might see Hermione's transformation as an abomination and try to 'correct' it or get rid of her. They could spread rumors about her and try to turn the wizarding community against her in her new form.
A plot could involve a shape - shifting creature that takes on the form of Draco and Hermione at different times. This causes chaos in Hogwarts as everyone is confused. Draco and Hermione must find a way to stop the creature and in the process, they discover secrets about each other and the magical world that they never knew before. They also start to see each other in a new light, leading to a possible romantic relationship.
One idea could be a story about a young wizard who discovers a new magical creature in the Forbidden Forest. Maybe it's a small, furry creature with the ability to turn invisible at will. The wizard tries to protect it from being discovered by other, less kind-hearted students or teachers.
One idea could be that Hermione turns into a Nunbu as a result of a magical experiment gone wrong. Maybe she was trying to study a new form of transformation magic and accidentally triggers this strange transformation. Then she has to figure out how to reverse it while dealing with the unique abilities and limitations of being a Nunbu.
One could be the Thestral. Since Hermione is very intelligent and curious, her discovery and study of Thestrals, which are invisible to those who haven't witnessed death, could be an interesting plot. Another could be the Niffler. Hermione might have an adventure trying to keep a Niffler from stealing shiny things in a museum or a wizard's home. And the Phoenix is also a great choice. Hermione could be involved in a story where she helps a Phoenix that is in trouble, perhaps injured or lost, given her kind-hearted nature.
One idea could be that when Hermione turns into a boy, she has to deal with different social dynamics in Hogwarts. For example, she might be treated differently by the boys in Gryffindor. Some might be more competitive with her, while others might befriend her in a new way. In the fanfiction, it could explore how she uses her now - male identity to her advantage in magic studies, perhaps being able to participate in all - male magical dueling clubs or something similar. And she could also have an interesting time trying to hide this transformation from her professors, especially McGonagall who is very perceptive.
One idea could be that Hermione turns into a centaur as a result of a misfired spell during a Defense Against the Dark Arts class. She could then struggle to adapt to her new form, having to deal with the physical differences like running on four legs and having a horse - like body. Meanwhile, her friends Harry and Ron would be trying to find a way to reverse the spell.
One idea could be exploring how the inheritance affects Harry's relationships with his friends. For example, if he inherits some special abilities from a creature, his friends might react with a mix of awe and concern.
You could write a fanfic about a magical creature that can travel through time. It appears to Harry and his friends during their fifth year. This creature is being hunted by a powerful dark wizard from the future who wants to use its time - traveling abilities to change the course of wizarding history in his favor. Harry and his friends must protect the creature and figure out a way to send it back to its own time safely, all while dealing with the normal chaos of Umbridge at Hogwarts.
Well, a plot could be centered around Draco the sub creature being wrongly accused of a crime in the magical realm. It has to prove its innocence while also dealing with the prejudice against sub creatures. Maybe there's a mystery element where it has to find the real culprit who is trying to frame sub creatures in general. Or, Draco could be part of a prophecy that only it can fulfill, but first it has to overcome many challenges and the distrust of other magical creatures.