One idea could be exploring the backstories of the minor characters in the 10th kingdom. Maybe a story about the life of a random villager and how the events of the main story affected them.
One idea could be that Wolf and Virginia get separated from the group again in a new magical land. Wolf has to use his wolf - like instincts to protect Virginia from unknown creatures. Another could be a story where they go back in time within the 10th Kingdom and have to change a certain event to save the future of the kingdom, all while their relationship deepens.
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.
A great plot could be about a female character from 'last kingdom fanfiction' who disguises herself as a man to join the army. She then has to constantly hide her identity while also dealing with the challenges of war and her growing feelings for one of her comrades.
Perhaps the 8th Hokage has to deal with internal political unrest within the Hidden Villages. Some ninjas might be unhappy with the new leadership and try to undermine the 8th Hokage's authority, leading to a complex web of intrigue and conflict.
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 idea could be a tribute who has a unique survival skill like being able to communicate with animals in the arena. They could use this to gather information about hidden resources or approaching threats.
One idea could be a revolution within the 100th Hunger Games. The tributes from different districts secretly plan to overthrow the Capitol's control over the games. They start by forming alliances not just for survival but to spread their message of resistance among the other tributes.