A fun plot could be a crossover with a detective - themed world. The Trinity Seven characters would have to use their magic to solve mysteries and crimes. Their magical abilities could provide new ways of gathering evidence and interrogating suspects that are not possible in a normal detective story.
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.
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.
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.
Another interesting concept is an OC who is from a completely different dimension but gets drawn into the Trinity Seven world. This OC could have a different set of values and magic rules from their own dimension, which would create a lot of interesting interactions and conflicts with the existing characters.
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.
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.
One idea could be a crossover with a popular fantasy series like 'The Lord of the Rings'. Imagine the characters from Trinity Blood in the Middle - earth setting, with vampires and other supernatural beings interacting with hobbits, elves, and humans. Another idea is a crossover with a superhero universe. The vampires in Trinity Blood having to face off against superheroes or even teaming up to fight a greater evil. For example, they could join forces with the Avengers.
A possible plot could involve the Trinity Seven characters trying to understand the chakra system from Naruto. They might see it as a new form of magic. Naruto himself could befriend some of the Trinity Seven characters and they could all go on a quest to stop a common enemy. The enemy could be something that threatens both the ninja world and the world of Trinity Seven, like an ancient evil that feeds on both chakra and magic.
One plot idea could be that the male OC discovers a hidden power within the Trinity Seven world that no one else knew about. This could lead to a race among different factions to either control or destroy this power. Another idea is that the OC has a connection to an ancient prophecy that involves the Trinity Seven characters. As he tries to figure out his role in the prophecy, he forms closer bonds with the group.
One interesting plot idea could be that in the Minecraft world, the characters from Seven Deadly Sins are transported there. For example, Meliodas could be on a quest to find a special diamond that has the power to restore his lost strength. And the Minecraft mobs could be influenced by the sins. The zombies could be more aggressive due to wrath, and the pigs could be gluttonous and overeat the crops in the game.