Perhaps a great plot idea would be to have a character who is an outsider to the Keyblade War but gets dragged into it. This character could bring a fresh perspective and different skills or abilities. They might try to find a way to end the war through negotiation rather than fighting, which would add a whole new layer to the story. And you could also explore how the power of friendship and trust can be a game - changer in the midst of all the chaos and battles.
One plot idea could be that Naruto discovers the Keyblade while on a strange new mission outside of the Hidden Leaf. He initially has no idea how to use it but through trial and error, he starts to unlock its powers. Another idea is that the Keyblade is a gift from a mysterious sage who believes Naruto has the potential to use it to save both the Naruto world and the world connected to the Keyblade. Maybe Naruto has to use the Keyblade to fight against an enemy that can manipulate time and space, which is a new and dangerous threat to his village and friends.
One idea could be Sora training a group of new male keyblade wielders. He could teach them his unique fighting techniques and the importance of the heart. It would be cool to see how he imparts his wisdom.
One interesting plot twist could be the unexpected alliance between characters who were previously enemies in the Keyblade War. For example, two factions that were fighting over the Keyblades might join forces to face a greater threat that emerges during the war.
Maybe there's a character who has the ability to traverse different universes freely in this 'the multiverse at war fanfiction'. Their journey to find a way to end the war by uniting the different versions of heroes from various universes would be really engaging. They might face challenges like different versions of the same villain trying to stop them.
A time - travel plot would also be great. Imagine a character from the future coming back to the time when Kaguya and Miyuki were just starting their 'war'. This new character could either try to help them get together faster or, for some reason, try to keep them apart. It would add a new layer of mystery and drama to the fanfiction.
A plot could revolve around a medic in the war trying to save as many casualties as possible. This could involve difficult decisions like who to treat first when resources are scarce. It would show the moral dilemmas in the chaos of war.
Well, for 'at war with nature fanfiction', a plot could involve a character who has a special ability to communicate with nature but is forced to use that ability to fight nature instead of befriend it. Maybe because some dark magic has corrupted the natural order. Or a story where an entire city is besieged by a horde of oversized insects and other creatures that are acting out of their normal behavior. The people have to figure out how to stop this onslaught and restore the balance. Also, a group of adventurers could accidentally awaken an ancient elemental force that now wants to destroy all human civilization as it views humans as a threat to nature.
Well, you could write a fanfiction where the characters discover an abandoned bunker full of supplies. But there's a catch - it's booby - trapped. This would lead to a lot of tense moments as they try to disarm the traps without getting killed. Also, a plot about a character who has a special skill, like being able to communicate with animals, and how that helps the group survive. Maybe they use the animals to scout for danger or find food sources.
A plot could revolve around a love story between a noble from one kingdom and a commoner from an enemy kingdom. Their relationship could be a catalyst for peace or more conflict. For example, they might try to secretly broker peace treaties while hiding their relationship from their respective kingdoms. This would add a layer of drama and secrecy to the fanfiction.
One idea could be a ment with special powers emerging and changing the power dynamics in the war. For example, a ment that can read minds and uses this to outwit the enemy. Another could be a group of ments from different factions coming together to try and stop the war, facing many challenges along the way. Maybe a ment discovers a hidden weapon that could end the war but has to decide if using it is the right thing to do.