If it's a crossover with a Naruto - like ninja world, Naruto himself would be a great addition. His energy and determination could contrast and complement the World Trigger characters. Also, Sasuke with his powerful ninja skills could add an interesting dynamic. In a crossover with a One Piece world, Luffy would be a fun character to include. His carefree yet strong - willed nature would bring a new flavor to the story.
One idea could be a crossover with a magical world. The World Trigger agents suddenly find themselves in a place full of wizards and magic spells. They have to adapt their combat skills to deal with magical creatures and powerful magicians. Another plot could involve a crossover with a post - apocalyptic world. The agents must use their Trigger abilities to survive in a world filled with mutated beasts and scarce resources.
Well, if we consider other shonen - style series, someone like Naruto from 'Naruto' could be a potential crossover character. His ninja skills and strong will could create some interesting dynamics with the 'world trigger' characters.
The character interactions can be really engaging. Characters from 'world trigger' might form unexpected alliances or rivalries with characters from the crossover universe. This can lead to new story arcs and development for all the characters involved.
One interesting concept could be the combination of different characters' abilities from the original Black Trigger universe in new and unexpected ways. For example, seeing how a character with a speed - based Black Trigger might team up with a power - based one in a crossover scenario.
Osamu Mikumo is quite popular. His growth as a character in the original series makes him a great focus in fanfiction. He can be written in various scenarios where he overcomes more challenges. Yuuma Kuga is also popular as his unique abilities and his mysterious past offer a lot of room for exploration in fanfiction.
One interesting concept could be alternative character backstories. For example, a fanfiction might explore what led a side character to become an agent. Another concept could be different rankings systems within the Border organization. It could also be about new types of Triggers that are created by the characters in the fanfiction.
First, you need to have a good understanding of the Black Trigger universe, including the characters, their abilities, and the overall setting. Then, decide which elements from different parts of the Black Trigger world you want to cross over. For example, you might choose to combine two different squads.