You can start by creating a scenario where the two worlds collide. Maybe it's a portal that opens up between the 'Seraph of the End' world and the other world. Then, introduce the main characters from both sides. Show how they react to the new and strange elements in each other's worlds. You could start with a character from 'Seraph of the End' stumbling into the other world by accident.
First, decide which other universe you want to crossover with. This will help you set the tone and the rules of the new combined world. Then, think about how the characters from 'owari no seraph' will interact with the new characters. For example, if it's a crossover with 'Sword Art Online', how would Yuichiro react to being in a virtual world? Next, plan out a basic plot. It could start with a strange event that pulls the characters into the new world.
Another idea is a crossover with a superhero series. The seraphs with their powers could team up with superheroes. They could work together to fight a common enemy that is threatening both their worlds. This would bring in a mix of superpowers and different combat styles.
Maybe in the crossover, a strange curse or magic affects both worlds and starts to merge them. Naruto and his comrades find themselves in a world filled with both demons and vampires. Yuuichiro and his group are also confused but quickly realize they need to work together. The shinobi could use their shadow clone jutsu to create distractions during battles against the vampires, while the vampires' superhuman strength and speed could be an advantage in the open combat scenarios in the Naruto world.
One common theme is power struggle. Since 'seraph of the end' has a complex power system among vampires, humans, and seraphs, when crossed with other universes, there is often a new power struggle. For example, if crossed with a superhero - based universe, who is more powerful - a vampire or a superhero? Another theme is survival. Both 'seraph of the end' and many other fandoms often deal with post - apocalyptic or dangerous situations, so survival becomes a key theme in crossovers.
To start writing a chara crossover fanfiction, you should start with brainstorming ideas for how the characters will interact. Will they be friends or foes at first? Also, think about how the different rules and worlds of the original characters will blend or clash. For example, if you're crossing a magical character with a non - magical one, how will that work? Sketch out a rough outline of the story, including the beginning, middle, and end. You can start with a simple premise like 'What if these two characters met?' and then build on it. Consider the character arcs for each of the crossed - over characters. How will they change during the story?
Begin by brainstorming ideas. Think about what would happen if the 'legion' met a different group of characters. You can draw inspiration from other crossover fanfictions you've read or movies/TV shows you've seen. After that, develop your characters. If you're bringing in characters from different universes, give them new dynamics. They might be initially suspicious of each other but then learn to work together. And don't forget to set the scene. Describe the new world that is created by this crossover, whether it's a mix of the 'legion' world and another or a completely new place where they all end up.
To start writing 'Battletech Crossover Fanfiction', begin by choosing a crossover that has potential for great stories. Say you pick 'Battletech' and 'Full Metal Panic'. Research both universes thoroughly. Next, create a strong plot. Maybe the mechs are being used in a war on a planet similar to the one in 'Full Metal Panic'. Develop your characters well, like how a Battletech pilot would react to the situations in the other universe.
Begin by exploring the personalities of Robin and the character he is crossing over with. If it's, say, Robin and Supergirl, understand how Robin's detective skills and combat training would interact with Supergirl's superpowers. You can start with a small encounter between them, like they meet by accident and then gradually build up to a bigger adventure. Also, don't forget to add some conflict, whether it's internal (Robin struggling with his own insecurities) or external (facing a powerful enemy).
Begin by creating a character profile for Android 21 in this new crossover setting. Consider her goals and how they might change. Also, establish the rules of the new world she's in. If it's a world of magic, how will her powers interact with magic? You can start with a simple scene, like her arrival in the new world, and build from there.
Kakashi and Guren could make for an interesting pair. Kakashi is a highly experienced and intelligent ninja, while Guren is a powerful vampire. Their different combat styles and knowledge could lead to some great interactions. Kakashi could try to figure out the secrets behind Guren's vampire powers, and Guren could be impressed by Kakashi's strategic mind and wide array of ninja techniques.