One idea could be that the reader is a new student at the Kingdom Hearts School. They're lost on their first day and a popular character from the series helps them find their way. This could start a friendship or even a budding romance.
Maybe the reader discovers a hidden power within themselves while at the school. This power attracts the attention of both good and bad characters. They then have to decide which side to trust as they navigate through the challenges at the school.
Perhaps there could be a tournament in High School DxD where characters from Kingdom Hearts also participate. The keyblade wielders would show off their combat skills against the devils and angels. This could lead to some interesting alliances and rivalries. The tournament could also be a cover for a greater evil that is trying to take over both worlds, and the characters from both universes have to figure it out and stop it together.
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.
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 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.
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.
Well, perhaps in this fanfiction, the Organization XIII has a plan to use Elsa's powers for their own evil purposes. They manipulate events in Arendelle to make Elsa feel isolated and out of control. Sora, Donald, and Goofy show up and befriend Elsa, showing her that she can trust others. They then have to battle the Organization XIII members while also dealing with the internal struggles of Arendelle's characters, like Anna's determination to save her sister no matter what.
Perhaps there could be a plot where the Organization XIII members from Kingdom Hearts have an interest in the Relics from RWBY. They start to manipulate events in the RWBY world to try and get their hands on these powerful artifacts. The RWBY characters, suspecting something is amiss, team up with Donald and Goofy who have also arrived in their world. Together, they have to figure out the Organization XIII's plan and stop them before they can steal the Relics.
Maybe there could be a story where the Keyblade wielders from Kingdom Hearts visit the world of FFXV to learn about their unique form of magic. They could then exchange knowledge, like how Noctis' warp strikes could be adapted into a new Keyblade ability.
One idea could be a crossover where the keyblade wielders from Kingdom Hearts end up in the world of Blade. They could team up with the Blade warriors to fight a new threat that is a combination of Heartless and the vampires from Blade. Maybe Sora discovers that his keyblade has new powers when used against the vampires.