A possible plot is a tournament - style story. In a magical multiverse tournament, Aqua and Erza are both participants. At first, they are focused on winning for their own reasons. Aqua wants to prove the strength of her Keyblade and her connection to light, while Erza wants to show the might of Fairy Tail. But as the tournament progresses, they find themselves having to work together against some underhanded opponents. They develop a friendship and respect for each other's abilities along the way.
Another plot could involve a quest to save their respective worlds. Erza discovers that there are strange magical disruptions in her world that are somehow connected to the Heartless in Aqua's Kingdom Hearts world. They embark on a journey together, during which they learn about each other's pasts, like Aqua's time in the Realm of Darkness and Erza's time in the Fairy Tail guild. This mutual understanding strengthens their bond and helps them succeed in their mission.
One idea could be a crossover adventure where Aqua and Erza team up to face a powerful dark force. Maybe they start off as rivals, with Aqua's Keyblade skills and Erza's swordsmanship clashing in a misunderstanding. But as they realize they share a common enemy, they combine their powers. Aqua could use her magic to support Erza's more direct combat approach.
One idea could be Aqua crossing over into a different Disney world, like Wonderland. She could have to navigate the strange rules and characters while still maintaining her Keyblade skills. Another could be a crossover with a Square Enix game universe other than Kingdom Hearts. For example, she might end up in the world of Final Fantasy and team up with some of those characters to face a new threat that somehow links both universes.
One idea could be a story about Aqua's training in the past. It could explore how she mastered her keyblade skills, the challenges she faced, and the friendships she made during her training days. Another could be a story where Aqua travels to different worlds in the Kingdom Hearts universe in search of something, like a rare magic item that could help her defeat the darkness. And a third idea could be a story that focuses on Aqua's relationship with her friends, like Terra and Ventus, and how they support each other through difficult times.
Aqua's Keyblade has the power to unlock hearts and manipulate the flow of light and darkness. Erza's power is very physical, with her various swords and armors. In a joint effort, Aqua could use her Keyblade to purify any dark energy that might be affecting Erza in a fight. Erza, in return, could use her physical strength and combat skills to protect Aqua while she's performing more complex magical maneuvers. Their powers would be a great complement to each other, creating a dynamic and interesting combination in the fanfiction.
One idea could be exploring the backstories of the new characters introduced in Kingdom Hearts 3 in more depth. For example, you could write about how they came to be in the different worlds and what their true motives are.
I'm not sure there is an official 'sexy' version of the Kingdom Hearts Aqua story. In the regular Kingdom Hearts, Aqua is a keyblade wielder. She is strong and brave. She embarks on a journey to protect her friends and the worlds from the forces of darkness. She has to face many challenges and enemies along the way.
One idea could be that the Heartless somehow find their way into the Marvel universe and start attacking various locations. The Avengers, being the protectors, team up with Sora and his friends from Kingdom Hearts. They initially have some clashes due to different fighting styles and ways of understanding the world, but eventually learn from each other and combine their powers to defeat the Heartless. For example, Iron Man could use his technology to analyze the Heartless' weaknesses, while Sora uses his Keyblade to directly combat them.
One idea could be that Sora finds himself in a new world where he is surrounded by characters from different Kingdom Hearts realms who all have a strong attraction to him. They compete in a friendly way to go on adventures with him.
Well, a great plot for a Kingdom Hearts 3 Aftermath fanfiction could involve the relationships between the different characters. Like, how Riku and Kairi's friendship might have evolved. Also, since Xehanort was defeated, there could be a power vacuum. Some villains might try to take advantage of it, and the heroes have to stop them before a new, greater evil rises. It could also show the daily lives of the characters in a more peaceful time, with little hints of the past adventures.
One idea could be exploring how the dalmatians' world is connected to the Kingdom Hearts universe in terms of magic. Maybe the dalmatians have a special kind of magic within them that can help Sora in his battles. Another could be a story where the dalmatians are being hunted by Heartless and Sora has to protect them. Also, perhaps there's a secret about the dalmatians' spots that has significance in the Kingdom Hearts lore.
One possible plot could be that Sora meets Aqua in a strange, dream - like world. They might initially be cautious around each other, but then realize they are on the same side in the fight against the Darkness. They could then team up to face some new threat that neither has encountered before.