Another interesting concept is to have the characters from Berserk interact with the magical creatures and races of the kingdom. The Band of the Hawk could be hired as mercenaries by the kingdom to fight off an invasion of some sort. This would lead to some great action scenes and also allow for character development as they interact with the new and different people of the kingdom.
One idea could be a Berserk and Dark Souls crossover. Picture Guts in the dark and unforgiving world of Dark Souls, facing off against the powerful bosses there with his greatsword. His tenacity and fighting skills would make for an exciting addition to the Dark Souls lore.
One idea could be that Guts from Berserk ends up in the world of Vinland Saga. He might cross paths with Thorfinn and they could either become allies or initially clash due to their different outlooks. Guts' brutal fighting style and his cursed brand could be a source of mystery and awe for the characters in Vinland Saga.
Maybe Sora, Donald, and Goofy end up in the Naruto universe. They would be really confused at first. Sora could try to understand the concept of chakra and how it's different from the magic he knows. They could team up with Naruto's team to take on Akatsuki members who have somehow been influenced by the darkness in the Kingdom Hearts world.
Well, a Berserk - Game of Thrones fanfic could focus on the concept of power. In Berserk, Griffith is obsessed with power and in Game of Thrones, many characters are vying for the Iron Throne. A fanfic could explore what would happen if Griffith was transported to Westeros. He would surely see the Iron Throne as a symbol of ultimate power and try to claim it. Or it could be about a character from Game of Thrones, like Tyrion Lannister, who has to deal with the nightmarish creatures of Berserk. He might use his wits to try and find a way to co - exist with them or defeat them.
To make it unique, focus on the character interactions. Don't just have the Berserk characters barge into the kingdom and start fighting. Let them form relationships, both positive and negative, with the kingdom's inhabitants. For instance, a Berserk character might fall in love with a local from the kingdom, which could change their perspective and goals.
One idea could be that Guts from Berserk is somehow transported to the Naruto universe. He could encounter Naruto and his friends. Maybe they initially mistake him for an enemy because of his intimidating appearance and weaponry. But as they get to know him, they realize his noble heart and his fight against the forces of evil in his own world, and they team up to face a common threat in the Naruto world.
One idea could be that Sora and his friends end up in Equestria. They could team up with the ponies to fight a new threat that is affecting both their worlds. Maybe the Heartless are somehow being drawn to Equestria's magic and they need to find a way to stop it together.
One interesting concept could be a crossover with the Final Fantasy series. You could have characters from Kingdom Hearts interact with iconic Final Fantasy characters in a new world. For example, Sora and Cloud could team up to fight a new threat. Another concept could be a crossover with Disney's Aladdin. Picture Sora and Aladdin going on a new adventure together in the world of the Magic Carpet. A third concept might be a crossover with the Legend of Zelda. Link could join forces with Sora to save both their worlds from a common enemy.
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.
One idea could be to have the characters from Kingdom Hearts end up in the world of Nier Automata. Sora could be exploring the post - apocalyptic landscapes, and his keyblade might react in strange ways to the technology and magic present in Nier's world. He could team up with 2B and 9S to fight against the common threats like the machines. The Heartless could also be introduced into Nier's world, creating new challenges for both the heroes of Kingdom Hearts and Nier Automata.