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.
Another interesting concept is to write a fanfic set in an alternate universe where the Justice League members have different powers or personalities. For instance, Superman could be a villain and Lex Luthor a hero.
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.
Maybe in the fanfic, the Justice League gets transported to the Code Geass universe. Batman could start analyzing the political and power structures there, while Wonder Woman is intrigued by the concept of Geass. They could team up with the Black Knights, with Flash helping with quick rescues during battles. And Aquaman might find some similarities between his underwater kingdom and the various factions in the Code Geass world.
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 to create an OC with the power to manipulate stone in unique ways. For example, they can turn themselves into a stone-like form for protection or use stone projectiles in combat. Another idea is to have the OC be a part of an ancient civilization that had a connection to the stones and now they join the Justice League to protect the modern world from threats related to those stones.
A cool plot could be that there is a rift in the multiverse that is causing chaos. Batman Beyond discovers it first and tries to get the Justice League's help. Along the way, they encounter alternate versions of themselves and have to work together to close the rift while also dealing with the differences and similarities between their various selves.
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.
A possible plot could involve a member of the Batfamily getting kidnapped and the Justice League helping to rescue them. During the rescue, they discover a secret weapon or technology that the villains were planning to use against the world, and they have to work together to stop it.
One idea could be that Batman discovers a hidden agenda within the Justice League that he believes endangers the world, so he decides to oppose them. For example, he might find out that a new technology the League is using has unforeseen and dangerous side effects.