In this fanfic, it could be that Eros is a transfer student from a different magical school. He meets Psyche, a shy but brilliant Hogwarts student. They are both chosen for a special task by Dumbledore, like exploring an ancient and dangerous part of the Forbidden Forest. Along the way, they discover secrets about themselves and each other. Eros might have a connection to an ancient magic that is related to love, which starts to affect his relationship with Psyche as they face various magical creatures and spells in the forest.
One possible main antagonist could be a dark wizard who is envious of Eros's powers or the relationship between Eros and Psyche. This dark wizard could be trying to disrupt their relationship and steal their magic for his own evil purposes. Maybe he was once rejected by a love, and seeing Eros and Psyche's love makes him want to destroy it.
One idea could be to explore Psyche's life before she meets Cupid. Maybe she has a premonition of their encounter in her dreams, which makes her curious about love from a young age. Another idea is to focus on Cupid's internal struggle. He's fallen in love with a mortal, which goes against the rules of the gods. He could be torn between his love for Psyche and his duty as Cupid.
Love is a very common theme. Since their story is a love story at its core, fanfics often explore different aspects of their love, like the initial attraction and the development over time.
Well, you could have a fanfic where Voldemort uses DND - style dark magic to open portals to other dimensions. The Order of the Phoenix has to quickly learn DND - style combat and magic rules to fight off the new threats that come through these portals. Harry could be sent on a special mission to find a DND - style mage who can teach them the counter - spells. Meanwhile, the Death Eaters are also trying to master this new form of magic for their own evil purposes. This would create a really exciting and unique story.
Perhaps a plot where the protagonist can choose to either side with the Ministry of Magic during a time of political unrest within the wizarding world or join a rebel group. This would involve different magical battles and moral dilemmas depending on the choice. In the 'harry potter cyoa fanfic', this could show different aspects of the wizarding society that we don't often see in the original books.
A cool plot for a Harry Potter Twilight fanfic could be a competition. The wizards from Harry Potter and the vampires and werewolves from Twilight compete in a series of challenges. These challenges could test both magic and the unique abilities of the Twilight characters. Or perhaps a love triangle between a Harry Potter character, a Twilight vampire, and a Twilight werewolf. It would be interesting to see how the different cultures and powers interact in matters of the heart.
Another plot could focus on the lives of the secondary characters from the original series. For example, Neville could be the head of Herbology at Hogwarts and discovers a new, dangerous plant that has some sort of connection to an ancient magic. The 'carry on fanfic' could then be about how he and his students deal with it, with some cameos from the main characters.
One idea could be that a dragon escapes from Gringotts and Harry has to use his skills to track it down. Maybe the dragon has a special connection to a powerful dark magic item that Voldemort was after.
One idea could be that Voldemort somehow enters the Runescape world to seek more power. Harry and his friends follow him there to stop him. They have to learn the rules of Runescape quickly to be able to fight Voldemort. Another idea is that a Runescape wizard discovers a portal to the Harry Potter world and brings some of the unique items from Runescape, like special runes, which causes a stir in the wizarding world.
A great plot for a Harry Potter academy fanfic might be a time - travel scenario. A group of students accidentally travel back in time to the founding days of Hogwarts. They have to navigate the different rules and attitudes of that era while also trying not to change history too much. Or perhaps there's a plot where a student is wrongly accused of a major crime at the academy and has to prove their innocence. They'll need to use their wits and the help of their friends to find the real culprit.