Perhaps there could be a story where Trinity is the only one who can understand Terumi's true motives and goals. Everyone else in the BlazBlue world sees him as a villain, but Trinity looks deeper. She tries to convince the other characters that Terumi isn't all bad, which causes a lot of conflict and drama in the story.
One idea could be that Terumi and Trinity are forced to team up against a common enemy. Maybe a new, powerful force emerges in the BlazBlue universe that threatens both of their interests. They start off distrusting each other but gradually learn to work together and in the process, develop an unexpected bond.
Perhaps a fanfiction where Terumi loses his powers and has to rely on his wits and the help of some unlikely allies. He could start to see the world from a different perspective without his powers, leading to character growth.
It can start with a chance encounter. For example, they meet during a chaotic battle. Terumi is initially intrigued by Trinity's power or her unique personality traits. This initial spark can lead to more interactions where they gradually learn more about each other.
A plot could revolve around a time - travel fanfiction. Maybe a character goes back in time and has to prevent an event that would have a catastrophic effect on the future of the Trinity Seven world. This could involve interacting with younger versions of the characters and changing their fates.
Well, a plot could revolve around Arata and Lilith being sent back in time to a crucial moment in the history of their magical world. They have to make sure that the events unfold correctly without changing the future too much. This would put a strain on their relationship as they might have different ideas on how to handle things. Another idea is that they find a mysterious prophecy that involves the two of them and they have to figure out what it means. As they search for answers, they encounter old enemies and new allies, all while their relationship evolves.
Another idea is that there is a mysterious illness spreading among the Trinity Seven members. Akio is one of the first to be affected, and Arata is determined to find a cure. He goes on a journey, visiting different places and asking for help from various magical beings. This fanfiction could explore Arata's determination and his growth as he faces numerous obstacles in his quest to save Akio. It could also show the bond between them as Arata refuses to give up on his friend.
A possible plot could be that there is a strange energy disturbance that starts to merge the three universes together. Characters like Ragna from Blazblue, the Persona users, and Team RWBY find themselves in a strange new world that is a mix of all three. They must figure out what is causing this and how to stop it. Maybe there are also new enemies that are created from the combination of the different mythologies and technologies of the three universes. For instance, Grimm with the ability to use Persona - like powers or Blazblue - style Ars Magus. It would be a great opportunity for character development as they learn to work together despite their differences.
Perhaps a crossover where the Trinity Seven characters enter a post - apocalyptic world. Their magic would be both a great asset and a source of danger as other survivors might see them as threats or saviors. They'd have to figure out how to survive and maybe even find a way to restore the world using their powers.
Jin could also be a great crossover character with Terumi. Jin's ice powers contrast with Terumi's dark powers. They could be pitted against a common enemy that requires them to combine their skills, which would be interesting to explore as they have different motives.
One idea could be that Naruto characters are transported to the Trinity Blood universe. Naruto, with his strong will and powerful chakra, might be seen as a threat or a potential ally in the complex political and religious landscape of Trinity Blood. For example, his shadow clone jutsu could be mistaken for some form of dark magic in that world.
A plot idea could involve a character from Highschool DxD getting lost in the world of Trinity Seven and having to adapt to their magic system. They could learn new spells and make new friends along the way, all while trying to find a way back home. It would be a great opportunity for character development and exploring the differences between the two universes.