Well, you could write a fanfiction where the different pantheons are in a sort of competition. Percy, being from the Greek pantheon, has to represent his side. He teams up with some unexpected allies from other pantheons, like a Valkyrie from the Norse pantheon. They have to complete a series of challenges set by the gods of all the pantheons involved. It would be full of action, with Percy using his water powers in new and creative ways against opponents from different mythologies.
One idea could be a crossover where Percy meets characters from the Norse pantheon. He could go on an adventure with Thor and Loki, and they have to face a common enemy that threatens both their worlds. Another idea is to have Percy enter the Egyptian pantheon's realm and learn their magic and ways, maybe even form an alliance with Anubis.
You can make it unique by focusing on the relationships between the different pantheons. For instance, show how the gods from the Greek and the Aztec pantheons view concepts like honor and sacrifice differently. Percy could be caught in the middle of these cultural differences and have to find a way to reconcile them.
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.
Perhaps in the fanfiction, Percy could experience a mental collapse. This could be due to all the pressure and trauma he has faced over the years. His friends might have to find a way to bring him back from this dark place, and in the process, we could see deeper relationships develop among them. For example, Annabeth could use her knowledge and their shared history to reach Percy on a different level.
Another concept could be that a powerful ancient evil is affecting both the Percy Jackson universe and Arendelle. Elsa and Percy are the only ones who can sense it at first. They must team up, traveling between their two worlds, to find the source of the evil. Along the way, they learn about each other's powers and cultures, and their combined powers are the key to defeating the evil.
One idea could be a crossover where Harry and Percy meet in a magical realm that combines the wizarding world and the world of Greek gods. They could team up to fight a common enemy, like a dark wizard who has also angered the gods.
One idea could be a fanfiction where a character gets a tattoo of Percy's sword, Riptide. The tattoo could have magical properties in the story, like glowing when danger is near. Another idea is a tattoo of the Camp Half - Blood logo. It could be a symbol of pride for a demigod character in the fanfic, and maybe it was given to them as a mark of their belonging to the camp.
Another interesting concept is a story where Percy Jackson is somehow transported to the Marvel universe. He could use his water - controlling powers in unique ways there. Maybe he'd join forces with Namor, the Sub - Mariner, and they'd have an underwater adventure together, facing off against some underwater threat that only their combined powers could handle.
In Ambrosia Percy Jackson fanfiction, one interesting idea could be new uses of ambrosia. For example, it might be used to heal not just physical wounds but also emotional traumas. Some fanfictions may explore how different characters react to ambrosia in unique ways. Like a new demigod who has never tasted it before might have a very different experience compared to Percy.
One idea could be that Percy can control not just water, but also the temperature of it. So he can freeze water instantly to create ice weapons or barriers. Another could be that he has enhanced underwater vision, allowing him to see clearly even in the darkest and murkiest depths, which would be really useful in his underwater adventures.