One idea could be exploring a new prophecy that involves Percy and his friends having to team up with some of the lesser - known gods. This would introduce new characters and powers into the story.
It's not likely that the actual Olympus (in the context of the Percy Jackson series being a place of the gods) reads fanfiction. Fanfiction is a creation of the fans in our world, and there's no indication in the books that the gods or Olympus as an entity would engage with it.
Reyna is another one. She's a strong and independent leader. Fanfiction could focus more on her past, her time leading the Roman camp, and how she copes with the different challenges she faces, especially when it comes to interacting with the Greek demigods.
A lesser - known but really interesting fanfiction is 'Annabeth's Quest for Wisdom'. This story follows Annabeth as she goes on a solo quest to gain more knowledge and wisdom. It shows a different side of her character, her determination and her thirst for knowledge in a very engaging way.
One great fanfiction is 'The New Quest of Percy Jackson'. It continues Percy's adventures in a very believable way, with new challenges and old friends by his side. The author really captures the essence of the original series.
Annabeth could definitely be one of his main allies. Her intelligence and strategic planning would be invaluable in a fight against Olympus. Also, Grover might be there, using his skills as a satyr to gather information and support from other nature - related beings. Some of the less - well - known demigods from Camp Half - Blood could also join in, seeing Percy as a chance for a new order.
His friends from Camp Half - Blood would be his allies for sure. Annabeth, with her strategic mind, would be invaluable. Grover, the satyr, could help Percy communicate with nature spirits which could be useful. But also, some of the minor gods who feel left out by the major Olympians might side with Percy. For example, Hecate could use her magic to assist Percy, seeing him as a chance for a new order in the Olympian world. There's also Tyson, Percy's cyclops half - brother, who would be a strong and loyal ally.
Perhaps the story involves Percy being framed as an assassin of Olympus. He then has to clear his name while dealing with the real assassin who is causing chaos among the gods and demigods. The plot might also include Percy's journey to discover who set him up and why.
In such a fanfiction, perhaps Percy could use his unique powers in new and unexpected ways. He might gain the support of some unexpected allies among the demigods or even some minor gods who are tired of the Olympian regime. This could lead to a large - scale battle where Percy's strategic thinking and combat skills are put to the ultimate test against the might of Olympus.
The fanfiction could explore the idea of the ninjas learning about the Greek magic and powers. They might be intrigued by the ability of the demigods to control water, fire, or other elements. On the other hand, the demigods could be amazed by the stealth and combat skills of the ninjas. This could lead to some really interesting training scenes where they exchange knowledge and techniques.