In a 'Trunks x Justice League' fanfic, it could be interesting if there was a villain who was trying to merge the two universes in a chaotic way. Trunks and the Justice League have to stop this from happening. Trunks uses his ki blasts and super - speed, while the Justice League members use their unique powers. Flash and Trunks could have a friendly race, which also helps them understand each other's abilities better. Aquaman could provide support in underwater battles, and together they all form a unique alliance to save both universes.
Perhaps Trunks is initially mistaken for a threat by the Justice League. But as they learn more about him and his mission to protect his own timeline, they start to work together. For example, Trunks could share his knowledge of advanced Saiyan fighting techniques with Batman, who then uses this knowledge to develop new strategies for the team. Meanwhile, Wonder Woman could teach Trunks about the different cultures and values in the Justice League's world.
One idea could be a tournament where the Z - fighters from Dbz and the Justice League members compete. For example, Goku could be paired against Superman in a friendly yet intense battle to see who is stronger in terms of physical strength and combat skills. Another plot could involve a common enemy that forces the two groups to team up. Maybe Darkseid and Frieza form an alliance and the combined forces of the Dbz characters and the Justice League have to stop them.
One idea could be X23 joining the Justice League on a mission to stop a powerful mutant - hating villain. Her unique combat skills and mutant abilities would add an interesting dynamic to the team.
One idea could be a story where Batman has to lead the Justice League in a fight against a powerful new villain who can manipulate time. This would put Batman's strategic skills to the ultimate test as he tries to coordinate the different powers of the League members. Another plot could be that a member of the Justice League is secretly under mind control and Batman has to figure out who it is before they cause too much damage.
One idea could be an OC with the power to manipulate time joining the Justice League. They could accidentally create a time rift during a battle, and the League has to fix it while also dealing with the OC's insecurities about their powers.
One idea could be that the Young Justice team has to prove themselves to the Justice League during a major alien invasion. The League initially doubts their abilities but as the battle progresses, they see the unique skills and teamwork of the young heroes.
One idea could be exploring the backstories of the lesser - known members in more detail. For example, Martian Manhunter's life on Mars before coming to Earth.
One idea could be that Broly and Trunks are forced to team up against a new, powerful enemy that threatens the entire universe. They start off not getting along because of Broly's wild nature and Trunks' more calculated approach. But as they face challenges together, they gradually learn to respect each other's strengths and form a deep bond.
One idea could be that a powerful cosmic entity threatens both universes. The Justice League and the Avengers initially clash due to misunderstandings in their attempts to deal with it. But then they realize they need to work together. They combine their powers and strategies, like Superman's strength with Hulk's brute force and Batman's intellect with Iron Man's technology. This could lead to epic battles and heroic sacrifices as they strive to save both universes.
Maybe in the fanfiction, the Hulk's gamma radiation has an unexpected effect on some of the Justice League members. For example, it could give Superman a new vulnerability or temporarily enhance the powers of Flash in a strange way. This leads to a complex situation where they need to both deal with the Hulk and the consequences of this new development.
One idea could be that the OC has a unique power that the Justice League initially distrusts but later comes to rely on. For example, the OC can manipulate time in small bursts, which at first seems dangerous but is actually crucial in stopping a villain who can freeze time.