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.
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.
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.
One idea could be exploring the backstories of the characters in the 'hopeless kingdom' within the Twilight universe. Maybe a character who was once a normal human in Forks but got dragged into the chaos of the hopeless kingdom. Another could be a love triangle between a vampire, a werewolf, and a human in this new and desolate setting.
One idea could be creating an OC who is a keyblade wielder from a forgotten world. This OC could be on a quest to discover the secrets of their world's connection to the greater Kingdom Hearts universe. They might encounter classic characters like Sora and Riku and have to prove their worth as a keyblade wielder.
One idea could be exploring the lost technologies of the ancient kingdom. Maybe the characters discover a hidden vault filled with advanced weapons or energy sources that were used in the past. Another could be delving into the political structure of the ancient kingdom, like if there were different factions or noble houses that vied for power.
One idea could be a story about a new cookie character arriving in the kingdom and having to prove their worth. They could start from the bottom, make new friends among the existing cookies, and together face a new threat that the kingdom has never seen before.
One idea could be a story where Accelerator has to protect Last Order from a new and powerful enemy. Maybe this enemy is after Last Order because of her unique abilities and Accelerator has to use all his power and strategic thinking to keep her safe.
One idea could be Raya and Sisu exploring new lands together after peace is restored in Kumandra. They could discover ancient secrets or new types of dragons.
Well, in a Naruto Sakura The Last Fanfic, a great plot idea might be that Sakura discovers a new medical jutsu that has the potential to change the way ninja heal. Naruto, of course, is there to support her. Maybe there's a sub - plot where they have to deal with some old enemies who are trying to steal this new jutsu for their own evil purposes. They then have to use their combined skills, Naruto's strength and Sakura's intelligence and medical skills, to defeat these foes and protect the new discovery.