Well, perhaps it's about a secret organization that uses the sewers as their base and an opposing force attacks them there. The story could explore the unique setting of the sewers, with its stench, narrow passages, and hidden chambers. There could be a battle for control over something valuable hidden in the sewers, like an ancient artifact or a powerful weapon.
In the 'attack in the sewers fanfic', it might center around a character who is lost in the sewers and suddenly becomes the target of an unknown assailant. The sewers could be depicted as a maze - like place, full of dead - ends and false paths. As the attack unfolds, the character has to use their wits and any available resources to survive. They might encounter other characters who are also trapped in the sewers, and together they form an alliance to fend off the attack.
It could involve a group of heroes or adventurers being attacked by strange creatures in the sewers. Maybe there are mutants or monsters lurking down there. They might have to fight their way through the dark and dirty tunnels, facing various traps and obstacles along the way.
It could be a group of street - smart kids who often explore the sewers and find themselves in the middle of an attack. Maybe they have their own little gang and they are used to the sewers' environment.
Perhaps in this fanfiction, Naruto lives in the sewers because he is hiding from some sort of threat in the village above. He might be wrongly accused of a crime he didn't commit. While in the sewers, he meets other outcasts or misfits who have also sought refuge there. Together, they form a sort of underground community. Naruto then tries to clear his name from the sewers, using the resources and the new friends he has made to gather evidence and prove his innocence.
The fanfic might involve a magic - user in the story. This person could use their powers to either directly attack the bandits, like casting spells to immobilize them, or use magic to aid the other attackers in some way, such as creating illusions to deceive the bandits during the attack.
One possible plot could be that the ponies accidentally end up in the Attack on Titan world through some sort of magical portal. They then have to figure out how to survive in this harsh environment. They could use their unique pony abilities, like Fluttershy's connection with animals, to try and understand the Titans better. Maybe they could even find a way to communicate with the Titans and find out if there's a way to stop the attacks without all the bloodshed.
Well, it could be about Ash and Gary being on an adventure when Gary suddenly has an asthma attack. Ash has to use his knowledge of Pokémon to find a way to help Gary, like finding a plant that has medicinal properties. Maybe they're in a forest and Ash sends out his Bulbasaur to search for the right plant.
Perhaps there's a new law or regulation in the superhero world that Spider - Man has broken, unknowingly. The Avengers, being the enforcers of sorts, attack him. Like, there's a rule about using certain powers in public and Spider - Man accidentally did it while saving people. The Avengers misinterpret his actions as blatant disregard for the law.
Well, it could be about the Survey Corps finding a crashed Cybertronian ship. Maybe the Titans are somehow related to a Decepticon experiment gone wrong. The Scouts would then team up with the Autobots to fight both the Titans and the Decepticons.
A great plot for such a fanfic could be that Mikasa's Titan form is a result of a connection to an ancient Titan power. As she becomes a Titan, she starts to have visions of the past, of how the Titans came to be. She then decides to use this knowledge and her new form to try and end the Titan - human conflict once and for all. Along the way, she has to face her friends who are initially scared of her but eventually come to accept her new form as she proves that she is still the same Mikasa deep down, just in a different body.
One possible plot could be that during Halloween in the Attack on Titan world, the Titans take on a more spooky appearance. Maybe they are covered in cobwebs and glow in the dark. The Survey Corps decides to use this to their advantage. They dress up as even scarier creatures to try and scare the Titans away from the walls, which leads to a series of intense and comical encounters.
Well, it could be about Eren Yeager somehow getting transported to the Naruto world. Maybe he teams up with Naruto to fight against a common enemy like the Akatsuki who have somehow gotten their hands on Titan powers. They would have to combine their unique skills, Eren's Titan transformation and Naruto's jutsu.