One plot idea could be that Harry Potter and Luke Castellan somehow meet in a magical realm outside of their own stories. Maybe they are brought together by a powerful magic that needs their combined powers to defeat an ancient evil. Another idea is that Luke, with his knowledge of Greek mythology, helps Harry understand some of the more mysterious aspects of magic in the wizarding world that have roots in ancient myths.
One idea could be a pre - series story where Thalia and Luke are training together in Camp Half - Blood and they stumble upon a mysterious prophecy that seems to involve them both. Another could be a story set during Luke's fall to the dark side, and Thalia tries to save him by appealing to their old friendship.
One idea could be a pre - series fanfic where Luke and Percy meet as kids before all the chaos. They could become friends briefly and that friendship could add an interesting layer to their later enmity. Another could be a fanfic where Luke is given a second chance after his fall and has to work with Percy to redeem himself. Maybe they have to go on a quest together to save someone important from a new threat in the demigod world.
One idea could be a story where Fem Percy and Luke team up against a new, powerful enemy that neither Camp Half - Blood nor Luke's former allies can handle alone. They have to put aside their differences and work together, with Fem Percy using her unique take on Percy's powers and Luke using his battle - hardened skills.
One common theme is redemption. Since Luke has made some bad choices in the original lore, many fanfictions focus on him trying to make up for his mistakes.
One great Harry Potter Luke Castellan fanfiction is 'The Path of Luke Castellan' on FanFiction.net. It delves deep into his backstory and his relationship with the world of Harry Potter in a really interesting way.
One idea could be that the scarecrow comes to life and becomes a magical creature that helps Harry Potter in a dangerous task. Maybe it has the ability to sense dark magic and warns Harry of approaching threats.
Perhaps Orochi is a hidden figure in the history of the wizarding world that has been forgotten. Harry discovers some old texts in the Hogwarts library that mention Orochi in relation to a long - lost magical artifact. This leads him and his friends on a quest to find the artifact before it falls into the wrong hands, as it has the power to either control Orochi or be used by Orochi to gain ultimate power over the wizarding world.
One plot idea could be that Harry and Angelina team up for a special Quidditch training session for the younger students. They discover a new flying technique together that gives Gryffindor an edge in future matches. Another could be that they go on a secret mission within Hogwarts to uncover a hidden artifact that has been causing strange magical disturbances. And a third idea is that Angelina gets injured during a match and Harry uses some of his unique magical skills, learned from his various adventures, to help her recover faster than normal.
One idea could be a story where a character has a special power to resist Dementors' effects more than others. Maybe they discover a hidden spell or have a unique magical ability that they use to fight off Dementors when others are in danger. For example, a young witch or wizard who can emit a positive energy that counteracts the Dementors' despair.
One idea could be Norberta getting lost in the Forbidden Forest and Harry has to find her. Another could be Norberta growing ill and Harry, with his knowledge of magical creatures, tries to heal her. Maybe Norberta could get into trouble with some other magical creatures and Harry has to step in to protect her.