A crossover with a superhero universe, say the Marvel Universe. Travis Touchdown from No More Heroes could find himself in New York City and get involved with the Avengers. Maybe he initially misunderstands their motives and gets into a fight with them, but then realizes they're on the same side against a greater threat.
How about a crossover with the Final Fantasy series? The colorful and magical world of Final Fantasy could be a great contrast to the edgy world of No More Heroes. Travis could enter a world filled with magic and different races, and have to adapt his combat skills to deal with magical creatures and powerful mages.
One idea could be to have heroes from different universes team up to fight a common, powerful enemy. For example, Superman from DC and Thor from Marvel joining forces.
A great concept for Harry Potter heroes crossover fanfiction would be to cross them with the characters from the Star Wars universe. Picture Hermione using her intelligence to outwit Darth Vader. Or Harry learning some Jedi mind tricks. Also, a crossover with the characters from the Percy Jackson series. The demigods and the wizards could have an interesting interaction. Maybe they could combine their powers to defeat a common foe that threatens both the magical and the mythological worlds.
One idea could be a crossover with the Naruto universe. The Saiyans could learn about chakra and how to use it, while the ninjas are amazed by the power of ki. Another could be a time - traveling plot where the Dragon Ball Heroes characters end up in the past of their own world and have to prevent a major catastrophe that changes the course of their history. Maybe a crossover with One Piece where they search for the One Piece together and face off against the powerful pirates and the Marines.
A cool plot for dc super heroes crossover fanfiction could involve Wonder Woman and Aquaman. Maybe there's a threat that endangers both Themyscira and Atlantis. They must combine their unique skills, like Wonder Woman's combat abilities and Aquaman's control over the sea, to save their respective homes. Also, a Martian Manhunter and Cyborg crossover could be interesting. The Martian Manhunter's shape - shifting and telepathy, along with Cyborg's technological prowess, could be used to stop a cyber - psychic threat.
A possible idea for a Fire Emblem Heroes crossover fanfic is to cross it with the Final Fantasy series. The characters from Fire Emblem Heroes could enter the world of Final Fantasy and encounter iconic summons like Ifrit or Shiva. They could also participate in battles against the likes of Sephiroth. Another option is a crossover with the Pokémon world. The Fire Emblem Heroes might befriend Pokémon and use them in battles, or they could be on a quest to stop an evil organization that's misusing Pokémon power.
One idea could be exploring the backstories of minor characters in more depth. For example, you could write about the early life of a demigod who was only briefly mentioned in the original series. Another idea is to create alternate endings to the main story arcs, like what if the heroes had made different choices during a crucial battle? You could also focus on the relationships between the characters in a different light, perhaps showing a friendship that was hinted at but not fully developed.
One idea could be a crossover with another popular animated show like 'Steven Universe'. OK KO could end up in Beach City and have to team up with Steven to fight off a new type of gem - infected villains. They'd combine their unique powers, like OK KO's power fist and Steven's shield, to take on the bad guys.
One interesting 'no more heroes fanfiction' could be about the backstory of a minor character. For example, exploring how they got involved in the game's crazy world.
One interesting concept could be Naruto characters teaming up with heroes from other universes. For example, Naruto and Spider - Man could combine their skills. Naruto's shadow clones could be used for distraction while Spider - Man uses his webs to capture enemies. Another concept might be the sharing of power - ups. If Naruto meets a hero with super strength, he could learn new ways to enhance his own physical attacks.
A really cool concept could also be the exploration of alternate universes within the 'Heroes of Olympus' setting. Fanfiction writers might create a world where the events of the original series took a different turn, and how that affects the crossovers with other elements. For instance, if a certain character didn't make a crucial decision in one way, how would that change their interaction with a character from another franchise in the crossover?