Well, a great plot for this crossover fanfiction could be a mind - control scenario. Some powerful psychic villain takes over the minds of several key members of the Justice League. The Young Justice team, being more in tune with the 'underground' and less - expected threats, notice the strange behavior first. They then have to work undercover within the Justice League's headquarters to find a way to break the mind control without being discovered. This would involve a lot of sneaking around, using their unique skillsets, and maybe even some young - led detective work. It could also lead to some really interesting character interactions as the Young Justice kids try to convince the unaffected members of the Justice League that something is wrong.
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.
For a Young Justice and Justice League crossover fanfiction, here's an idea. A cosmic event causes a rift in reality that starts to merge different parts of the DC universe in strange ways. The Young Justice team finds themselves in the middle of this chaos, with some of their members getting mixed - up powers or abilities. The Justice League is also dealing with the fallout, but they approach it from a more traditional, experienced perspective. The two teams must come together, with the Young Justice bringing their fresh ideas and the Justice League their wisdom. They could also explore new relationships between characters, like a young hero getting mentored by a different Justice League member than they usually would, which could add depth to the story.
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 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.
A cool plot could be a time - travel related crossover. The Young Justice team travels back in time and meets the younger versions of the Justice League members. They have to work together to stop a time - traveling villain from changing history in a way that would destroy the future. Or perhaps there's a magic - based crossover where a powerful sorcerer is causing chaos. The Justice League, which is more used to physical threats, has to team up with the Young Justice, some of whom have magic - related powers, to defeat the sorcerer.
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.
A crossover with the Avengers. Picture the Justice League and the Avengers having to work together to stop an interdimensional threat. There would be clashes in leadership styles between Captain America and Superman, and also a lot of cool moments as different power - sets are combined to fight the enemy.
A cool plot could be a race between The Flash and Gotenks. They both pride themselves on their speed, so it would be interesting to see how that pans out. Also, perhaps Vegeta could befriend Batman as they both have that brooding, serious personality. They could exchange combat and strategy tips. And what if the Dragon Radar was used by the Justice League to track down an energy source that was causing chaos in both universes?
One idea could be that the Covenant from Halo discovers Earth while the Justice League is busy dealing with a local threat. The League has to quickly adapt to this new alien threat that has far more advanced technology than most of their foes. They might have to team up with Master Chief, who is also on a mission to stop the Covenant's invasion plans.
Well, a cool plot could be that some sort of cosmic event has merged their universes in a strange way. Members of the Avengers start showing up in the Justice League's world and vice versa. Iron Man could team up with Batman to figure out how to reverse the situation while Thor and Superman have an epic showdown that ends in mutual respect. And then they all work together to fix the messed - up universes.