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.
The plot could involve a powerful ancient artifact that is being hunted by both supervillain groups. The Avengers and the Defenders initially compete to get it first, but then realize they need to work together. Captain America's leadership can bring order to the combined team, and the Hulk's brute strength, along with the Thing's, can be used to fight off the villains while the more intelligent members like Black Panther and Mr. Fantastic figure out how to use the artifact to save the day.
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.
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.
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 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.
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.