One idea could be exploring the backstory of the main character's family in more detail. Maybe there were secrets or hidden powers within the family that were never fully explored in the game.
A fanfic could also focus on the relationship between different characters. For instance, delving deeper into the friendship or rivalry between two characters. You could create side stories that show their interactions outside of the main quest, perhaps during their downtime or in secret meetings.
A great plot could be centered around a prophecy that involves both universes. In the Percy Jackson side, there could be a prophecy about a hero from another world (Final Fantasy) coming to help defeat a new and powerful monster. Meanwhile, in Final Fantasy, there could be a similar prophecy about a hero with strange powers (Percy). And so, characters from both worlds start to search for each other and then work together to fulfill the prophecy.
One interesting plot idea could be a world - merging scenario. Where the world of Cardcaptor Sakura and a Final Fantasy world start to blend, and Sakura has to use her card - magic to navigate the new and dangerous landscapes filled with Final Fantasy creatures.
One idea could be exploring the backstories of the minor characters in more depth. For example, the story of the civilians in Cocoon who were affected by the events. Another could be a what - if scenario, like what if Lightning had made different choices at certain key moments. And a third could be focusing on the relationship between Fang and Vanille before they became l'Cie.
A crossover with the Star Wars universe could also be exciting. Imagine the characters of Final Fantasy 13 in a galaxy far, far away. Snow could be like a Jedi, using his strong will and combat skills, while Fang might be more of a bounty hunter type, roaming the galaxy in search of valuable prey.
A great plot idea could be an OC that is a former villain who has seen the error of their ways. They now try to redeem themselves by helping the heroes. For example, they might have insider knowledge about the plans of the main antagonist. Another plot could be an OC who is a healer with a very unorthodox method of healing. They are initially scorned by the traditional healers but prove their worth during a major battle. Also, an OC who is a thief with a heart of gold, stealing from the corrupt nobles to help the poor and then getting involved in the main story's quest could make for an exciting fanfiction.
A really interesting plot could be the reader having the ability to communicate with the fayth in a unique way. This could give the characters in Final Fantasy X an edge in understanding the secrets of Spira and how to break the cycle of Sin. It could also lead to some philosophical discussions between the reader and the characters about the nature of sacrifice and the afterlife as it relates to the fayth and the summoners.
A possible plot is that Fem Harry is a hybrid of a human and a Final Fantasy creature. This makes her an outcast in both worlds. She then embarks on a journey to find a place where she truly belongs. Along the way, she meets a group of rebels in the Final Fantasy world who are fighting against an oppressive regime. Her skills and courage make her a valuable asset to the group, and together they strive to change the world for the better.
One plot idea could be that Naruto is somehow transported to the world of Final Fantasy 13. There, he meets the characters and due to his unique abilities and charm, a harem starts to form around him. He could use his ninja skills to help the characters in their battles against the fal'Cie. Another idea is that the characters from Final Fantasy 13 are brought to the Naruto world. The different cultures and powers collide, and as the FF13 characters adjust, they are drawn to Naruto, creating a harem situation. The harem members could then team up with Naruto to protect both worlds from common threats.
One idea could be a crossover with another popular RPG like Dragon Quest. The characters from World of Final Fantasy could end up in the world of Dragon Quest, facing new enemies and having to adapt to different magic systems. For example, the mirages in World of Final Fantasy might interact in strange ways with the slimes in Dragon Quest.
One idea could be a male self - insert who has the power to control time. He uses this power to prevent certain tragic events in the Final Fantasy 7 world, like the destruction of Aerith. Another plot could involve the male SI being a master of a new type of materia that no one has seen before, which attracts the attention of both Shinra and Avalanche. He has to decide which side to ally with or if he should remain neutral.