I'm not entirely sure specifically as it depends on the source material it's crossing over with. It could be a fan - created story that combines elements from the '91 Days' anime or manga with another franchise. Maybe it features characters from '91 Days' in a new and different setting or plot that's inspired by the other work it's crossed with.
One idea could be to cross Dog Days with a popular magical girl anime. For example, the characters from Dog Days could meet and team up with the Sailor Scouts. They could combine their powers to fight a new, powerful enemy that threatens both their worlds. This would bring a fresh dynamic as the Dog Days characters' combat styles are quite different from the magical attacks of the Sailor Scouts.
Sorry, I don't have specific '91 days crossover fanfiction' to recommend directly. But you can start by looking on popular fanfiction websites like Archive of Our Own or FanFiction.net. Search for '91 days crossover' and then sort through the results based on ratings and reviews.
First, decide which two universes you want to cross. If it's Dog Days with another anime or series, study both thoroughly. Know the characters, their personalities, and the rules of their respective worlds. For example, if crossing with a high - fantasy world, understand the magic systems. Then, come up with a basic plot. It could start with a simple idea like a character from Dog Days accidentally ending up in the other universe.
Naruto would probably be very enthusiastic. He would be eager to make friends with the characters in Dog Days. His outgoing personality would make him approachable. The characters in Dog Days might be initially cautious but soon warm up to him. For example, the princess in Dog Days could be intrigued by Naruto's stories of his own world.
Well, it could be really interesting. Maybe Naruto and his friends somehow end up in the world of Dog Days. Naruto might be confused by the different rules and the animal - like characters. He could use his ninja skills in unique ways there. For example, his shadow clone jutsu could be used to create distractions during battles in Dog Days' world. And his determination to protect his new friends in this strange land could drive the story forward.
I'm not sure specifically what '310 days fanfiction' is about as there could be many different fanfictions with such a title. It might be a story that spans over a period of 310 days within the context of a particular fandom, perhaps following the characters' growth, relationships, or adventures during that time.
Another idea is that Greninja gets exposed to some kind of meta - human - creating substance in the DC universe and gains new powers. It could then join the Teen Titans, sharing its unique Pokémon abilities with the group. It would be interesting to see how Greninja interacts with characters like Robin, Starfire, and Cyborg, and how it uses its powers in battles against the Titans' enemies.
In this fanfiction, Skynet could find a way to connect to the Matrix. It might try to take over the Matrix and use it to control more humans. The machines in the Matrix might initially resist, but Skynet could offer them a deal to share power over humanity.
One popular genre is probably the drama - filled ones. Since '91 days' itself is full of intense drama, fanfictions in this genre would further explore the emotional turmoil of the characters. They might focus on the internal struggles of characters like Nero, who is caught between his loyalty to different parties.