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.
Perhaps the Avengers are framed for a crime they didn't commit. Sherlock, being the master of deduction, decides to take on the case. He starts by interviewing each Avenger, using his skills to determine their innocence. Along the way, he discovers a hidden conspiracy that involves a powerful enemy trying to discredit the Avengers so they can take over the world.
One idea could be to have Sherlock investigate a mystery related to a historical event in Hetalia. For example, he could look into the disappearance of a valuable artifact during a time of war in one of the Hetalia countries. Another idea is to have the Hetalia characters assist Sherlock in solving a case in London. Maybe Italy could provide some unexpected insights with his unique perspective. Also, a crossover could involve Sherlock being transported to the Hetalia world and having to adapt to its strange rules and cultures while still using his detective skills to solve local mysteries.
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.
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.
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 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 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.
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.
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.