A really cool plot idea for a Kingdom Hearts Infinity War fanfiction would be a crossover of the two worlds in a more complex way. Picture the Guardians of the Galaxy meeting the Kingdom Hearts gang. They could exchange stories about their different universes. Then, when Thanos starts collecting the Infinity Stones, it causes some sort of rift in the Kingdom Hearts world. This rift makes it possible for some of the villains from Kingdom Hearts, like Ansem, to team up with Thanos in exchange for power. The heroes from both sides then have to figure out how to stop this new and powerful alliance.
Well, a plot idea could be that the Infinity Stones have some sort of connection to the Kingdom Hearts universe. Maybe they are fragments of Kingdom Hearts itself. So, the characters from Kingdom Hearts have to race against the Avengers to either protect or claim the stones. Also, some of the Disney characters from the Kingdom Hearts world could get involved. For example, Mickey Mouse could use his magic to help in the fight against Thanos and his forces.
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.
Well, you could write a fanfiction about a group of keyblade wielders who get trapped in a strange, distorted version of their own world. They have to figure out how to escape while also dealing with their own personal demons and secrets. As they explore this new and dangerous place, they learn more about themselves and the true nature of their powers, which could lead to some interesting character development.
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.
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.
One idea could be that the main character from 'The Gamer' gains the power to level up in the 'Kingdom Hearts' world. So they start leveling up their keyblade skills like it's an RPG. They could also gain stats related to magic, speed, and strength, which would change how they interact with the Heartless and Nobodies.
One idea could be that Tangela gets transported into the Kingdom Hearts universe and has to team up with Sora to find a way back to its own world. Along the way, Tangela discovers new powers that are a combination of its Pokemon abilities and the magic in the Kingdom Hearts world.