Well, how about a story where the Organization XIII members from Kingdom Hearts: Birth by Sleep infiltrate the Naruto world? They could try to manipulate the different ninja villages, causing chaos. Naruto and his friends would have to figure out their motives and stop them. Meanwhile, some of the Naruto characters might start to develop abilities similar to the Organization XIII members. For instance, Sakura could gain some sort of power related to Xion's ability to copy others' forms, but in a way that is unique to the Naruto universe's ninja skills.
Another interesting concept is an OC that has a special connection to the darkness but is trying to fight it. This OC could start as a character who is feared by others in the 'Kingdom Hearts' universe because of their association with the dark side, but as the fanfiction progresses, they prove their worth and loyalty to the light. Maybe they were experimented on by the villains, which gave them this connection to the darkness.
It mainly follows the stories of Ventus, Terra, and Aqua. They are Keyblade wielders. Their adventures involve battles against the forces of darkness, exploration of different worlds, and their individual quests for self - discovery and growth within the complex Kingdom Hearts universe.
The plot mainly focuses on the backstories of the key characters in the Kingdom Hearts universe before the events of the main games. It delves into their relationships, their quests, and how they are all connected to the overall Kingdom Hearts story.
A cool concept might be the fusion of different storylines. Maybe the fanfiction combines the story of 'Kingdom Hearts Birth by Sleep' with another related or even unrelated game's story. This can lead to unique plot twists and character developments. For instance, if it crosses over with a Final Fantasy story, we could see how the magic systems of both interact and how the characters adapt to the new rules.
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.
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.
Sasuke would also be a great choice. His dark past and his search for power could parallel some of the characters in Kingdom Hearts: Birth by Sleep. He might have an interesting dynamic with characters like Terra. Sasuke could be both intrigued and challenged by the power of the Keyblades and the different magic systems in the Kingdom Hearts universe. His own Sharingan abilities could also create some really cool interactions when combined with the powers in the Kingdom Hearts world.