Perhaps in the fanfiction, a cosmic event causes a rift between the Marvel and DC universes. Captain America and the Justice League find themselves on the same side as they are the only ones who can prevent the destruction of both universes. They must overcome their differences in ideology and power - sets. Captain America's sense of justice and the Justice League's diverse powers and abilities would make for an interesting dynamic as they work together to close the rift and defeat the forces trying to destroy the universes.
One plot idea could be Captain America joining the Justice League temporarily. He could bring his military tactics and sense of honor, and have clashes and then reconciliations with Batman over leadership and strategy. For example, when facing a powerful alien invasion, Captain America might advocate for a more direct, all - out attack while Batman prefers a more stealthy, long - term plan.
I think Captain America and Batman would have some great interactions. Batman is the dark and brooding detective, while Captain America is the all - American hero. They could clash over their methods of fighting crime at first. Batman might be skeptical of Captain America's more straightforward and moral - centered approach, but over time they could learn from each other and form a strong partnership.
One possible plot could be that Naruto is somehow transported to the DC Universe. He teams up with the Justice League to stop an evil force that is using a combination of Naruto - style ninjutsu and advanced technology from the DC world. For example, Lex Luthor could be the mastermind behind this, having found a way to harness chakra and is using it to create powerful and dangerous weapons. Naruto's skills in combat and his ability to sense and manipulate chakra would be a great asset to the Justice League, and in return, the League could teach Naruto about their powers and technology.
One possible plot could be that Percy Jackson's world is suddenly invaded by a powerful threat that the demigods alone can't handle. So, Percy and his friends seek the help of the Justice League. The League initially doubts the existence of Greek gods and demigods but soon realizes the seriousness of the situation. They team up, with Percy teaching the heroes about the magic and monsters of his world while they teach him about modern combat and technology. Together, they fight the new threat that combines both magic and advanced weaponry.
It could be really interesting. Steve Rogers' sense of justice and leadership would blend well with the Justice League. His military tactics could help in planning battles against powerful villains. For example, in a fight against Darkseid, Steve might come up with a strategy to use the different powers of the League members effectively, like having Flash create distractions while Superman goes for a direct attack.
One possible plot could be a threat that is too big for either the Justice League or the X - Men to handle alone. So they have to team up. Maybe a powerful cosmic entity is threatening to destroy both the mutant and non - mutant worlds. The heroes from both groups would need to combine their powers and strategies. For example, Superman's strength and flight could be paired with Wolverine's combat skills and healing factor. Batman's intellect could work with Professor X's telepathy to come up with a battle plan.
One possible plot could be that the Omnitrix gets malfunctioned and starts sending out energy signals that attract the attention of the Justice League. Ben, in his various alien forms, has to work with the League to figure out what's wrong with the Omnitrix and stop an evil force that is also drawn to the energy. For example, Vilgax could team up with some of the League's villains like Lex Luthor to take advantage of the situation.
In a fanfic, Captain America could fit into the Justice League by being the voice of the common man. The Justice League often deals with big, cosmic threats, but Captain America would remind them of the importance of protecting the ordinary people on the ground. His experience in World War II gives him a unique perspective on the cost of war and the value of peace, which could influence the Justice League's decisions when it comes to using force and resolving conflicts.
One idea could be Captain America arriving on Themyscira and being initially met with suspicion by the Amazons. But as they learn of his values of justice and courage, they start to respect him. He could then train with the Amazons, sharing his combat techniques while also learning from their unique fighting styles.
One possible plot could be that the Pokemon and Justice League members have to team up to stop an evil force that is trying to take over both the Pokemon world and the cities protected by the Justice League. Maybe a powerful, shadowy villain has discovered a way to use Pokemon powers in the human world in a very destructive way, and only by combining the unique abilities of the Pokemon and the superpowers of the Justice League can they hope to defeat this new threat.