Another interesting plot could be a sort of competition between the two groups that turns into a forced alliance. Like, both teams are after the same powerful artifact. But then they realize that a villain is also after it to destroy both their worlds. So they have to put aside their differences and work together. The Hulk could team up with Luke Cage for some brute - force action against the bad guys.
A cool plot could be a race against time. There's a threat that will destroy the world in a matter of days. The Avengers and Defenders initially clash over how to handle it, but then realize they need to combine their resources. For example, Doctor Strange from the Defenders could use his magic to enhance Iron Man's technology for a more powerful weapon against the enemy.
One idea could be that a powerful cosmic entity forces the Avengers and the Defenders to team up. They have to combine their unique skills, like the Avengers' high - tech gadgets and teamwork, with the Defenders' more street - level and magic - based powers to defeat this entity. For example, Doctor Strange from the Defenders could use his magic to create portals for the Avengers to quickly move around in battle.
Definitely Daredevil and Black Widow. Daredevil's enhanced senses and martial arts skills combined with Black Widow's espionage expertise and combat abilities would make for an exciting duo. They could work together on a stealth mission, with Daredevil using his senses to detect hidden threats and Black Widow using her gadgets and charm to get them in and out of dangerous situations.
Doctor Strange and Thor would be an interesting pair. Thor with his Asgardian magic and brute strength, and Doctor Strange with his powerful sorcery. They might have some initial clashes over whose magic is more powerful, but then come to respect each other and use their combined powers to defeat a powerful magical enemy.
One idea could be that the Avengers get transported to a Disney fairy - tale world. For example, they end up in Arendelle from 'Frozen'. Iron Man could try to figure out how to adapt his suit to the cold, while Captain America tries to lead the way through the icy terrain. Thor might find similarities between the magic in Arendelle and Asgardian magic.
Another concept could be that one of the NCIS agents has a family member who is in danger because of a supervillain's plot. The Avengers get involved to protect the agent's family, but the NCIS team doesn't trust them at first. Gibbs would be very protective and suspicious. As the story progresses, they all learn to work together. Ziva could show off her combat skills to Black Widow, and they could have a friendly sparring match while sharing their combat experiences.
One idea could be that Oliver Queen gets transported to the Avengers' universe. He could teach them some of his stealth and archery skills. Maybe he helps Hawkeye improve his arrow techniques. Another idea is that the Avengers come to Star City. They help Oliver fight a new, super - powered villain that is too much for him to handle alone. The Avengers' powers and teamwork could combine with Arrow's street - level fighting style in an interesting way.
One idea could be that Merlin uses his magic to help the Avengers in a battle against a powerful magical enemy. Maybe he disguises himself as a normal human at first and then reveals his true powers when the situation is dire.
One idea could be a plot where the Avengers and DC heroes are brought together by a powerful cosmic entity that threatens both universes. They have to combine their powers and strategies to defeat it. For example, Superman and Thor could work together to take on the entity's brute strength while Batman and Iron Man figure out its weaknesses using their intellect.
A great concept for Marvel Avengers crossover fanfiction is to have the Avengers cross paths with the Fantastic Four. The two teams could collaborate on a scientific project that goes awry, creating a new threat that only their combined powers can defeat. Another possibility is a crossover with the Eternals. Since the Eternals have been on Earth for a long time but hidden, their discovery by the Avengers could lead to a complex relationship. They might initially be at odds, but then realize they need to work together to stop a greater evil that is awakening on Earth. Also, an Avengers crossover with Blade would be interesting. Blade could seek the Avengers' help in fighting a new breed of vampires that are more powerful than ever and have ties to some dark magic that the Avengers have to figure out.