One idea could be that a powerful cosmic threat emerges that neither the Justice League nor the Avengers can handle alone. So they have to combine their forces. For example, Superman and Thor could team up to take on a super - strong enemy, using their strength and lightning powers respectively. Batman and Iron Man might start off butting heads as they both think they have the best strategies, but eventually work together to outwit the villain's high - tech defenses.
Perhaps a powerful magical item has been stolen and it's up to the magic - users from both teams, like Doctor Strange and Zatanna, to find it. Along the way, they encounter various challenges. The Flash and Quicksilver could have a friendly race while also scouting for danger. This could also lead to some interesting character interactions as the different heroes learn from each other's abilities and fighting styles.
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.
Maybe a villain from the Justice League's rogues gallery teams up with an Avengers foe. Let's say Lex Luthor and Doctor Doom. They create a device that can swap powers between heroes from both teams. This leads to some really fun and chaotic situations. For instance, Captain America with the Flash's speed or Wonder Woman with the Hulk's strength. The two teams have to figure out how to work with their new, swapped powers and stop the villains.
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.
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 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.
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.
One idea could be that a powerful cosmic entity forces them to fight. It creates an artificial situation where they believe the other group is a threat to the universe. For example, it manipulates their perceptions so the Avengers think the Justice League is stealing powerful artifacts to control the multiverse, and vice versa.
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.
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.