Well, you could have a fan fiction where the Justice League has to deal with a reality - warping threat. This would make their world start to merge with other dimensions, and they have to find a way to stop it while also dealing with their own changed powers and identities. Also, a story about the League's origin from the perspective of an ordinary citizen who witnesses their formation and how it changes the world around them could be really cool. Maybe they see Batman recruiting the first members and the challenges they face in the early days.
A great plot for Justice League fan fiction could be that a long - lost Kryptonian artifact surfaces on Earth. This artifact has the power to either save or destroy the planet, depending on how it's used. The Justice League must race against time to find it first and figure out its purpose before their enemies do. In the process, they discover secrets about Superman's home planet that could change everything. They also have to deal with various villains who are also after the artifact, leading to intense battles and strategic planning.
One idea could be an alternate universe where the Justice League members have different powers. For example, Superman could lose his flight but gain super strength tenfold. Another could be a story where they face an enemy that can manipulate time, forcing the League to constantly adapt and outwit the foe. Or a plot where a new member, a powerful but untested hero, joins the League and causes internal strife as they try to fit in and prove themselves.
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.
An interesting plot could be that a new villain emerges who has the power to steal powers. He first targets the Miraculous heroes, taking away their powers one by one. When the Justice League realizes what's going on, they offer their help. But the villain then targets them too. So, the powerless Miraculous heroes and the weakened Justice League members have to find a way to work together. They search for ancient artifacts that can restore their powers and defeat the villain.
One idea could be that the Young Justice team gets accidentally transported to an alternate dimension where the Justice League has turned evil. The Young Justice members then have to figure out how to reverse the situation while also evading capture from their once - heroes. Another could be a time - travel plot where the Young Justice team travels back in time and teams up with the early days of the Justice League to stop a threat that was thought to be long gone but has resurfaced in their own time. A third idea is that a powerful new villain emerges who can only be defeated by a combined effort of the Young Justice and Justice League, but first they have to overcome their differences in fighting styles and leadership.
One idea could be a misunderstanding that leads to a battle. Maybe a villain manipulates the situation so that Young Justice thinks the Justice League has turned evil. For example, the villain uses mind - control technology on some of the League members and Young Justice has to fight to save them.
One idea could be a misunderstanding that leads to a fight. Maybe the Justice League mistakes some of Young Justice's actions as being too reckless and end up in a battle. Then they realize they were wrong when a real threat emerges and they have to work together.
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 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.