What are some interesting plot ideas for a 'holy world war fanfic'?

One idea could be a story where the inklings and octolings have to put aside their differences during a world war - like situation. They could form an alliance to fight against a common enemy that threatens both of their species. Maybe this enemy is a new type of sea creature that has been mutated by some sort of toxic waste and has a powerful army.

A third idea is to create a story where there is a technological race in the war. One side is developing a super - weapon in the form of a powerful new aircraft. The other side's pilots have to figure out a way to stop it. It could involve a lot of dogfights and strategic planning.

One idea could be introducing a new type of Servant, like a Servant from a different mythology not previously seen in the Grail Wars. This would bring in new powers and dynamics. Another could be having a master who is an ordinary person thrust into the war by accident and has to quickly learn the ropes. For example, a school student who stumbles upon the Grail War's secrets and has to fight to survive.

One idea could be a role - reversal plot. Instead of the usual power struggles between Kaguya and Miyuki, have them switch personalities for a day. It would be hilarious to see Kaguya being overly confident and forward, while Miyuki is shy and hesitant.

One idea could be to focus on a young recruit's journey through the war. Show their initial fear and inexperience, and how they grow into a capable warrior. For example, they might start out being afraid of the chaos demons but learn to face them bravely.

One idea could be a war between different Vocaloid factions over control of the music industry. Each faction has its own unique style and they battle it out in epic musical duels.

One idea could be a story about an underdog kingdom that discovers a long - forgotten magic and uses it to turn the tides of the war. Another could be a fanfic centered around the love story between a knight from one kingdom and a princess from an enemy kingdom, which then affects the course of the war. Or perhaps a tale of a young strategist who rises through the ranks and devises brilliant battle plans for his kingdom in the War of Kingdoms.

One idea could be a story about an underdog kingdom rising to power through unexpected alliances. Maybe a small, resource - poor kingdom teams up with a group of powerful but outcast mages. Another could be a love story between a warrior from one warring kingdom and a noble from an enemy kingdom, which could potentially change the course of the war.

A fanfic could center around the idea that the girl they were fighting over in the movie actually has a secret identity as a spy. When they find out, they have to team up with her to take on a bigger threat, all while still dealing with their feelings for her.

One idea could be focusing on an underdog character in the Clocktaur War. This character could start as a simple peasant who gets caught up in the chaos. Through a series of events, they discover a hidden power within themselves that could potentially change the tide of the war. Maybe they find an ancient artifact that has the ability to control the Clocktaurs to some extent.