One idea could be exploring the backstories of the minor characters in the War of the Last Alliance. Maybe there was a young soldier who had a secret love interest back home and his journey was not just about the war but also about getting back to her.
A really interesting plot could be a story from the perspective of an enemy within the alliance. Maybe there was a character who was being blackmailed by the opposing side to leak information. This would create a lot of tension and mystery as the other characters try to figure out who the traitor is and stop them before it's too late.
You could write about a love story set within the context of the war. For instance, two soldiers from opposing sides who fall in love. Their relationship would be filled with secrecy, danger, and moral dilemmas as they try to navigate their feelings while being part of the war.
One idea could be exploring the untold stories of the soldiers on the front lines. Maybe focus on a small group who are trying to complete a secret mission during the chaos of the time war. Another could be delving into the origin of the time war and how it could have been prevented from the perspective of a time - traveling historian.
One idea could be that the alliance forms due to a common threat from a new, even more powerful enemy. For example, a third alien race arrives on Sera and starts to decimate both human and locust populations. The two groups are forced to put aside their differences and combine their unique combat skills and technologies to fight this new foe.
One idea could be that the last dragon is discovered by a young adventurer. They form an unlikely bond and together they set out to find a way to save the dragon's kind from extinction. Maybe there are ancient prophecies or hidden magic involved.
One idea could be exploring an alternate timeline where Aang fails to defeat the Fire Lord. Maybe in this timeline, the Fire Nation completely takes over the world, and the remaining benders have to go into hiding and plan a long - term resistance. Another plot could be focusing on a new airbender discovered long after Aang's time, and their journey to master airbending without the guidance Aang had.
A cool plot for 'the last kingdom fanfic' might be to have a character be transported back in time to a crucial moment in the kingdom's history. They would have the knowledge of what's to come but struggle to convince others to believe them. This could lead to all sorts of interesting twists. Or you could write about a character who is secretly training a group of women to be warriors in a society that doesn't really accept that. It would be about breaking gender norms and the challenges they face in a war - torn world.
Gil - galad, the Elven king, is another main character. His elven wisdom and power were important in the fight against the enemy. His interactions with the human and dwarf leaders in the alliance would be a key part of the story, as they had to work together despite their differences.
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.
Another plot idea could be a war that the hounds are involved in goes awry. They might face an enemy they were not prepared for, and they have to use their unique abilities in new and unexpected ways to survive. For example, if they usually rely on speed, they might have to use their intelligence more in this new situation.
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.