I like the headcanon where Annabeth is a master of disguise. In fanfiction, she can blend into any crowd, not just because of her intelligence but also because she has some magical items that help her change her appearance. It makes her even more resourceful on quests. Grover, on the other hand, has a headcanon where he can grow plants with just a touch. So if they're in a barren area, he can create a little oasis for them to rest in. This shows his connection to nature in a more powerful way.
One interesting headcanon could be about the backstories of the Servants. For example, there might be a headcanon that a certain Servant had a secret friendship in their past life that influenced their character in the game. Another could be related to the magic system in the Fate Grand Order universe, like a unique way of casting spells that isn't shown in the main story.
One idea could be that Neville Longbottom is secretly a powerful wizard all along but has been suppressing his power due to self - doubt. Another could be that the Hogwarts portraits have their own little society and gossip network within the castle walls. And a third one might be that Hagrid has a secret family of magical creatures in the Forbidden Forest that he takes care of.
One interesting fanfiction could be about Sally's early adventures before Percy was born. It might explore her relationship with Poseidon in more depth. Another could focus on a day in the life of Sally and Percy when they have a special family event, like a reunion with other demigods at Camp Half - Blood. And there are also fanfictions that rewrite certain events from the original series with a different perspective from Sally's point of view.
In fanfiction, one interesting 'Percy Jackson moment' could be when Percy has to face a new and unexpected monster that wasn't in the original books. For example, a monster that combines the powers of multiple Greek mythological creatures. This makes for a really exciting battle scene where Percy has to use his wits and powers in new ways.
Well, time - travel is a cool concept in percy jackson fanfiction. Imagine Percy going back in time to change some crucial events in the history of the demigods. Also, there are fanfictions that focus on Percy having new powers or abilities. This could be because of some new magical artifact or a special training he undergoes. And of course, the idea of Percy having to adapt to a completely different camp, like a Roman - Greek hybrid camp, is really engaging.
One idea could be a fanfiction where Percy and his friends go on a new quest in a modern, urban setting. Instead of the usual mythological locations, they have to deal with monsters hiding in big cities. Another idea is a story that focuses on the relationships between the characters in a more in - depth way, like exploring the friendship between Percy and Grover during their off - quest times. And a third idea could be a time - travel fanfiction, where Percy gets sent back to the time of the ancient Greek heroes and has to interact with them while still holding onto his modern values.
One idea could be to explore Percy's life after he has fully mastered his powers. Maybe he goes on a quest to find a long - lost relative who also has demigod powers. Another could be a story where Percy and his friends have to deal with a new type of monster that is immune to their usual weapons and they have to find a creative way to defeat it.
One interesting concept could be a darker version of Percy's powers. For example, instead of just controlling water, he might be able to use it in more menacing ways, like creating water blades that can cut through almost anything. Another concept could be a darker side to his relationships. Maybe his friendship with Grover takes a turn, and there's more conflict and distrust. Also, a darker Percy might have a different view on the gods, perhaps feeling more resentful towards them for all the hardships he has faced, instead of just being the hero who blindly follows their orders.
Some fanfictions might explore what would happen if Percy had a different parentage. Maybe he was the son of a different god or goddess, like Hecate. This would completely change his powers and the way he interacts with the magical world around him. He could learn new types of magic and have a whole new set of challenges and adventures.