A training exercise gone wrong. The two groups could be participating in a joint training session organized by the Justice League. However, an unexpected power malfunction or a mind - controlling device causes the two teams to turn on each other. This would lead to some intense battles and also opportunities for individual characters to shine as they try to figure out what's really going on.
One idea could be a misunderstanding that leads to a battle. Maybe some misinformation makes the Batfamily think Young Justice is involved in something illegal or dangerous. For example, a new villain frames Young Justice by using their tech to carry out crimes. The Batfamily, being the protectors of Gotham, jumps into action without fully investigating.
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 a misunderstanding that leads to a battle between the two teams. Maybe a villain manipulates information so that the Young Justice team believes the Titans are up to no good. Then, as the fight unfolds, they start to realize they've been tricked.
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 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 great plot for batfamily crossover fanfiction could be that they cross paths with other superhero families from different universes. For instance, they meet the Fantastic Four and have to combine their skills to stop a common enemy. Or perhaps they find themselves in a world where all the supervillains have joined forces and they need to form alliances with unlikely heroes. Maybe a crossover where they enter a virtual reality world created by a tech - savvy villain and have to fight their way out.
Tim Drake from the Batfamily might be likely. He's highly intelligent and sometimes over - thinks situations. If he discovers some unusual activity related to Young Justice that he misinterprets, he could be the one to start the ball rolling on a conflict. For example, if he sees some new Young Justice members using untested tech that he thinks could be dangerous, he might take preemptive action against them.
Well, a cool plot could be time - travel related. Say a device sends members of the Avengers back in time to the DC universe and they accidentally disrupt something that makes the Justice League think they're invaders. Or perhaps there's a villain who has the power to manipulate minds on a large scale and he turns the two teams against each other. Also, a plot where the two teams have to compete in a series of challenges set by an ancient being for some great reward, but the challenges are designed to make them fight each other.
A possible plot idea is that Batman discovers a dark secret within the Justice League that they are unaware of. This secret could be something that could potentially destroy the League or change their moral compass. Batman then has to decide whether to confront the League directly or try to fix the problem on his own, which causes the 'vs' situation as the League may see his actions as a threat.