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.
In an 'Avengers Venom crossover fanfiction', perhaps Venom's host could be a new character related to one of the Avengers. For example, the son of Captain America who somehow gets bonded with Venom. This would create a whole new dynamic within the Avengers. The team would be conflicted between their loyalty to Cap and their fear of Venom. Meanwhile, Venom would have to learn to co - exist with the Avengers' values and not just go on a rampage.
One idea could be a Venom crossover with Spider - Man in an alternate universe where they have to team up against a common enemy like Carnage. Another could be a Venom and Deadpool crossover, with their crazy personalities clashing and then somehow working together. Maybe a Venom crossover with the X - Men, where Venom's alien nature causes some interesting reactions among the mutants.
I think Spider - Man would be really interesting. Since in the comics, Venom has a long history with Spider - Man, and if you bring that into the Avengers' world, it could add a whole new layer of drama. The Avengers would be caught in the middle of their relationship, not fully understanding the complex history between them.
Another interesting concept is an OC who is a scientist researching Venom's origin. They accidentally get infected and use their new powers to help the Avengers in a unique way. For example, they could have enhanced senses that allow them to detect threats before the Avengers even know they're there. They also have to struggle with the dark side of the Venom symbiote while trying to be a hero.
Perhaps Venom could cross over with the Guardians of the Galaxy. Venom's dark and edgy nature would contrast well with the Guardians' more light - hearted and space - faring adventures. They could be forced to work together to stop a powerful alien threat that is drawn to Venom's symbiote powers.
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.