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.
Maybe X23 is sent to the DC universe by some unknown force and she has to figure out how to get back while also getting involved with the Justice League's affairs. For example, she could be mistaken for an enemy at first and then prove herself to be an ally.
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.
One idea could be a crossover where the Young Justice team has to team up with the Justice League to face an interdimensional threat. Maybe a powerful villain from another dimension is trying to invade Earth and the combined skills and powers of both groups are needed to stop it.
Maybe a story where some of the younger members of the Young Justice team are frustrated with the Justice League's methods and decide to take on a mission on their own, but end up getting in over their heads and have to call in the League for help. This could lead to a lot of character development and some interesting dynamics between the two groups.
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 that the Justice League members gain new gaming - related powers. For example, Superman could have the ability to instantly analyze any opponent's weaknesses like a game character stats screen. Batman could use his detective skills to hack into virtual reality systems within the story's universe.
One idea could be a story where Batman has to lead the Justice League in a fight against a new, powerful and mysterious villain that can manipulate time. Another could be a fanfiction exploring the relationships within the Justice League when they are stranded on an uninhabited planet and have to rely on each other more than ever.
Perhaps in the fanfiction, there is a parallel universe where the Justice League are all vampires. They are trying to protect both the vampire and human populations from a greater evil that threatens to destroy both. They face challenges in getting the human - dominated Justice League in our universe to trust them and work together.