A really cool concept would be if Percy, while left behind, enters a different dimension or parallel world. In this new world, he has to adapt to different rules and face new challenges. This could lead to some really exciting adventures and character development as he tries to find his way back to his original world and reunite with his friends.
Perhaps Percy is left for dead in a magical trap set by an old enemy. As he tries to escape, he befriends a strange creature that he initially thought was an enemy but turns out to be a key to his survival. He has to rely on his wits and newfound friendship to get out and seek revenge on those who left him.
A possible plot is that Hermione is accidentally left behind in a different time period. She has to adapt to the ways of that time while also trying to find a way back to her own time. During this, she might meet some interesting characters from the past, and her knowledge of magic from the future could either help or put her in more danger.
Annabeth would be a great character to include. Her relationship with Percy is so strong that she would surely be trying to find a way to get him back. Their bond could add a lot of emotional depth to the story.
One interesting concept could be the merging of different character personalities. For example, if a character from 'Left Behind' is crossed over with a character from another series, their contrasting beliefs and values can create engaging storylines. Maybe a character who is very religious in 'Left Behind' meets a more skeptical character from another story, leading to debates and growth for both.
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 plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
One possible plot could be Naruto's journey of self - discovery as he tries to come to terms with being left behind. He might find hidden strengths within himself. Another could be him forming unexpected alliances with other characters who also have a sense of being alone. For example, he could team up with a wandering ninja who has his own story of abandonment.
One idea could be a crossover with a post - apocalyptic zombie series. The characters from Left for Dead could find themselves in a different zombie - infested world and have to adapt to new threats and survivors. For example, they might encounter a group that has a very different approach to survival, like a religious cult that believes the zombies are a form of divine punishment. This could lead to conflicts and alliances as the Left for Dead characters try to find their way back to their own world.
One idea could be a crossover with the Harry Potter universe. Percy could visit Hogwarts and have to deal with both magical and mythological threats. Maybe he'd team up with Harry to defeat a common enemy that combines dark magic and ancient Greek monsters.