One idea could be a story where Chain and Leo have to team up to solve a mystery in the hidden world of Hellsalem's Lot. They might start off not getting along well due to their different personalities, but as they face various challenges together, they gradually learn to trust and rely on each other.
Another fanfiction could revolve around a scenario where Leo gets into some sort of trouble that only Chain's unique abilities can help him out of. Maybe he accidentally uncovers a dangerous secret and Chain has to use her chains to protect him while they figure out how to keep the secret from falling into the wrong hands. This could also lead to some deeper emotional connection between them as they share this intense experience.
Maybe Chain is revealed to be working for an organization that has ulterior motives regarding Leo. However, as the story progresses, she realizes she doesn't want to go through with their plans and instead decides to protect Leo at all costs. This would be a great way to show her character development and also create a lot of drama in the story.
One idea could be exploring the backstories of the minor characters in Kekkai Sensen. For example, you could write about the life of a random citizen in Hellsalem's Lot and how they deal with the chaos. Another idea is to create an alternate universe where the main characters have different powers or personalities. Maybe Leo has the power to control time instead of his usual ability.
One plot idea could be that Leonardo and White get involved in a mystery together within the Kekkai Sensen world. They might have to uncover a secret organization's plot that endangers the city. Another could be a story where they go on a journey to find a lost artifact that has special powers related to their world. Maybe they start off not getting along but as they face challenges, they grow closer.
One possible plotline could be the characters from Kekkai Sensen getting involved in the Holy Grail War from Fate. Maybe they are summoned as Servants or they interfere with the war for their own reasons. Another could be the Fate characters ending up in the world of Kekkai Sensen and having to deal with the strange creatures and the kekkai barriers there.
One idea could be a crossover where characters from Naruto and Kekkai Sensen meet and have to team up to face a new, powerful enemy. Maybe they need to combine their unique abilities like Naruto's jutsu and the abilities of the Kekkai Sensen characters.
One idea could be a crossover with another popular anime like 'My Hero Academia'. The characters from Kekkai Sensen could enter the world of My Hero Academia and have to deal with the different power systems and challenges there. For example, Leo could try to understand how quirks work compared to his own abilities.
The Kekkai Sensen manga has indeed ended. It provided a satisfying conclusion to the adventures and characters it presented throughout its run.
Another interesting angle could be a story set in an alternate universe where the abilities in Naruto and Kekkai Sensen are combined. Maybe some characters inherit both ninja techniques and the special powers from Kekkai Sensen. This could lead to all sorts of new and exciting battles and character development as they learn to control and utilize these combined powers.
One idea could be to create an OC who is a powerful sorcerer from a different dimension and has to adapt to the unique rules of the kekkai sensen world. They could bring new types of magic or abilities.
Searching for 'Kekkai Sensen x Fate fanfiction'? One approach is to join fanfiction communities or forums related to either Kekkai Sensen or Fate. There, you can ask fellow fans if they know of any good crossovers. Another tip is to look for fanfiction that combines the unique elements of both series well. For example, if Kekkai Sensen has some interesting magic systems and Fate has its own set of heroic spirits, a great fanfiction would blend these aspects in an engaging way. Also, don't be afraid to try out different search terms related to the characters or concepts in both series to widen your search results.