One idea could be to cross the Avengers with the X - Men. Imagine Wolverine and Captain America teaming up for a mission, their different leadership styles would create interesting dynamics. Another could be crossing them with the Harry Potter universe. Picture Iron Man trying to figure out how magic works in Hogwarts. Or a crossover with the Justice League, seeing Thor and Superman in a battle of strength would be epic.
A cool Avengers crossover fanfic idea is to bring in characters from the Star Wars universe. For example, having Hawkeye and Han Solo on a heist together. They both have that roguish charm and sharpshooting skills. Also, a crossover with the Doctor Who universe could be fascinating. The Doctor could end up helping the Avengers time - travel to fix some alternate reality problems. Maybe they could even meet some of the Doctor's enemies along the way.
One idea could be that the Avengers have to deal with a supernatural force that is causing chaos in the city. Maybe it's a powerful demon that they can't defeat with their usual weapons and tactics. They then seek help from the Supernatural world, like Sam and Dean Winchester, who teach them about exorcisms and using holy water.
A really interesting plot could involve the Tesseract from the Avengers. It could be that the Tesseract has some sort of connection to the Darkness in SPN. When the Avengers are trying to protect the Tesseract, they accidentally release some of the Darkness. The Winchesters, with their knowledge of the supernatural, and the Avengers, with their superpowers, must then join forces to contain the Darkness again. The characters could also have some really fun interactions, like Dean being in awe of Iron Man's technology and Steve Rogers being impressed with Sam and Dean's hunting skills.
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.
One idea could be that Voldemort discovers the Infinity Stones and tries to use their power to conquer both the wizarding world and the Muggle world. The Avengers, with the help of Harry Potter and his friends, must stop him. They could combine magical spells with superhero powers to create unique battle strategies.
One idea could be that Voldemort teams up with a powerful Marvel villain like Thanos. This would force the Avengers and the Harry Potter characters to join forces. The Avengers could bring their strength and high - tech weapons, while the Harry Potter characters use their magic. Another plot could be that a magical artifact from the Harry Potter world is lost in the Marvel universe and both groups have to search for it. Maybe it has the power to destroy both universes if misused.
One idea could be that Voldemort somehow makes an alliance with Loki. The Avengers would then have to team up with Harry, Hermione and Ron to stop them. They could combine magic and superpowers in battles. For example, Iron Man could use his technology to analyze and counter some of the dark magic spells while Harry uses his Patronus to protect the group from Loki's illusions.
One idea could be a battle against a common enemy that is too powerful for either the Avengers or the X - Men alone. For example, a powerful alien race that wants to enslave mutants and humans alike. The two groups would have to combine their unique powers and skills to defeat this enemy. Another plot could involve a misunderstanding between the two teams that leads to a brief skirmish before they realize they are on the same side. For instance, the Avengers might mistake the X - Men's actions as a threat to humanity when in fact they are trying to protect a mutant from a rogue government agency.
Perhaps there could be a time - travel related plot. A device is accidentally activated in a S.H.I.E.L.D. lab that sends some Agents and Avengers back in time. They end up in a crucial moment in history, like during World War II. There, they meet younger versions of some of the Avengers. They have to work together without changing the timeline too much while also finding a way back to their own time.
One idea could be that a powerful cosmic entity threatens both the Avengers' world and the X - Men's. So they have to combine their forces. For example, Iron Man could work with Beast to develop new tech to counter the entity's energy. Another could be a mutant - hating villain who uses some of the Avengers' tech to target mutants, forcing the two groups to team up to stop him.
One idea could be that a powerful mutant villain emerges who is able to manipulate time. This causes disruptions in both the X - Men and Avengers' universes. The X - Men, with their knowledge of mutants, and the Avengers, with their vast resources and superpowers, have to team up to stop this villain. They start off distrusting each other, especially because the Avengers are sometimes seen as anti - mutant by some in the X - Men world. But as they face challenges together, they learn to work as one.