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.
Definitely Ben and Batman. Batman would be intrigued by Ben's Omnitrix and his ability to transform into different aliens. He would try to analyze it while Ben would be in awe of Batman's detective skills and gadgets. It would be a clash of two very different mindsets - one a young, impulsive hero and the other a brooding, strategic one.
One possible scenario in 'Ben 10 meets the Justice League fanfiction' is that a new villain emerges who has the power to nullify the powers of the Justice League members. Ben 10, with his Omnitrix, which has a vast array of alien powers, becomes the key to defeating this villain. His alien forms like Heatblast could provide the necessary firepower against the villain's cold - based defenses, while Four Arms could take on the villain in hand - to - hand combat if needed. The fanfiction could explore how they combine their strategies and powers to emerge victorious.
In a Ben 10 and Justice League crossover fanfiction, there could be epic battles. Ben could use his alien forms to team up with Superman and take on powerful villains. For example, Ben as Four Arms could help Superman in a physical fight against a huge and strong enemy.
Another interesting plot could be that a powerful villain creates a device that can steal powers. He targets both Ben's aliens and the Justice League members. Ben and the League have to find a way to stop the villain before they lose all their powers. They go on a wild adventure across different dimensions to find the parts needed to destroy the power - stealing device.
One possible plot could be that the Omnitrix malfunctions and sends Ben 10 to the Justice League's universe. The Justice League initially views Ben as a threat due to his alien forms. But as they learn more about him and the threat of an inter - dimensional villain, they team up. Ben's different alien forms offer unique skills that complement the League's powers, and together they fight the villain to save both universes.
Some members might be shocked. Wonder Woman, for instance, who respects Batman's skills and tenacity, would likely be very disappointed. She might try to persuade him to stay, emphasizing the importance of his presence in the team. Superman, on the other hand, might initially be understanding, as he knows Batman has his own way of doing things. But he would also be concerned about the power vacuum that Batman's departure would create. Aquaman might be more indifferent, thinking that Batman was always an outsider to some extent, with his focus mainly on land - based issues.
In fanfics involving Catwoman in the Justice League, she often brings a unique dynamic. Her morally grey nature contrasts with the more traditional heroes. For example, she might be involved in a heist that has unexpected consequences for the Justice League. Her skills as a thief and her acrobatic abilities make her a formidable character in these fictional stories.
In fanfiction, they might interact due to a misunderstanding at first. Maybe the Justice League sees Ben using his alien powers and thinks he's a threat. But then they learn that there are other forces at work that are endangering both their worlds. So they start to communicate and plan together. Ben can show the League his knowledge of different alien races and technologies, while the League can share their experience in dealing with big - scale threats on Earth.
The Highbreed could also serve as the main villain. They are a powerful and xenophobic race in the Ben 10 universe. In a crossover, they might view the Justice League's world as a place to expand their territory. They could use their advanced technology and large numbers to attack. Ben would have to convince the Justice League that the Highbreed are a serious threat and together they would need to find a way to stop the Highbreed's invasion, perhaps by using Ben's alien forms like Upgrade to hack into their technology and the Justice League's strategic planning.