Perhaps a villain from a Disney movie teams up with an Avengers' foe. Maleficent could join forces with Loki. The Avengers would then have to deal with two powerful magic - users. This would create a very interesting dynamic as they'd have to find ways to counter both types of magic.
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.
There could be a fanfiction where Loki, the trickster from the Avengers, ends up in Arendelle and causes chaos. He could manipulate some of the characters or try to steal Elsa's powers for his own gain. And on the other side, the Avengers are searching for him. Meanwhile, the Frozen characters have to deal with Loki's mischief on their own until the Avengers arrive. This would create a lot of interesting scenarios and character interactions.
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.
One idea could be Venom bonding with Hulk. Imagine the power boost and the chaos they could cause together. Another is Venom facing off against Thor. The contrast between Thor's godly powers and Venom's alien abilities would make for an exciting battle. Also, Venom could team up with Black Widow and use his stealth and her combat skills for some undercover missions.
One idea could be that Harry uses his magic to help the Avengers in a battle against an otherworldly threat. For example, he could create magical shields to protect the team or use spells to disorient the enemy. Another idea is that the Avengers visit Hogwarts and learn about the magical world. Tony Stark might be fascinated by the magic technology and try to find a way to combine it with his own inventions.
Another idea is that Sherlock and Tony Stark team up. They are both highly intelligent. Tony could provide Sherlock with high - tech gadgets to aid in his investigations, while Sherlock could help Tony with his personal problems. For example, Sherlock might figure out who is trying to steal Stark Industries' secrets by observing the small details in the company's operations.
One idea could be that Oliver Queen from Arrow gets transported to the Avengers' universe. He could teach them some of his unique archery skills and in return, the Avengers could show him advanced technology like Stark's Iron Man suits. Maybe they team up to fight a common enemy that has the ability to manipulate time, causing chaos in both their worlds.