Perhaps a fanfiction where a Solarballs character has a long - lost relative on Neptune. They go on a journey to Neptune to find this relative and end up uncovering secrets about the planet and their own family history. Along the way, they form alliances with other beings on Neptune who help them in their quest.
It could also be a sidekick character from Solarballs who is often overlooked but has some hidden talents. On Neptune, they get the chance to shine. Maybe this character is really good at solving puzzles or has a special ability to communicate with different forms of life, which becomes crucial on Neptune.
You could write about the Son of Neptune getting lost in a mysterious realm. In this realm, the rules of his world don't apply. He has to learn new ways to survive and fight. Along the way, he meets a group of misfits who he has to team up with. Together, they try to find a way back home while also uncovering secrets about this strange place that could have implications for his own world.
Well, a great plot for Percy son of Neptune fanfiction could involve Percy getting lost in a time - warp within the ocean. He could end up in an era where Neptune was just starting to gain his powers. This would force Percy to interact with a younger version of his father and also deal with the different rules and threats that existed back then. Additionally, Percy could find a way to bring some knowledge from that time to his present to help in a current conflict. Also, Percy might have to team up with some primitive sea creatures who have their own unique abilities and motives.
A really cool concept would be to set the fanfiction in a different time period. Imagine the Son of Neptune in the modern world facing challenges that are unique to our time, like dealing with technology that is both a help and a hindrance. Or you could place them in a historical setting, like during the Roman Empire, and see how their powers and identity play out in that context.
One plot idea could be Percy discovers some hidden prophecy while reading 'Son of Neptune' that affects his future adventures. Another could be that he reads it and starts to question his own role in the demigod world more deeply.
One idea could be Percy forming an unexpected alliance with a minor Roman god. In the fanfiction, this god could have a unique power that helps Percy in his quests. For example, a god of illusions who can create decoys to mislead Percy's enemies.
A really interesting plot would be if Percy was transported to a parallel universe where the roles of the Greek and Roman gods were reversed. He would have to adapt quickly to this new world, make new friends, and fight against the enemies who now have different motives and powers. It would be a great opportunity for character development and exploring different mythologies.
One plot idea could be that the OC guy is a new hero in the game world where Neptune exists. He starts off as a bit of an outcast but Neptune sees his potential. They go on adventures together to save the game world from a new virus threat. As they journey, they start to develop feelings for each other through shared hardships and victories.
Another interesting plot could involve Female Neptune being from a different dimension and having to adjust to the world of RWBY. She could bring her own set of values and skills from her home dimension, which could cause some clashes and also some great character development moments within the team.
Another plot could involve Percy's journey to discover more about his Roman heritage. He could be sent on a quest by Neptune himself to find an ancient artifact that is key to the future of the Roman demigods. Along the way, he has to deal with the politics and rivalries within the Roman camp. Some might see him as a threat to their power, while others see him as their only hope. He also might have visions of his past lives as a Roman hero which could guide him on his quest.