One common theme is the clash of different power levels. For instance, when the Defenders cross over with more powerful cosmic characters from the Marvel universe, like the Silver Surfer. There's also the theme of cultural differences. If they cross over with characters from a different fictional world, like a Japanese anime world, cultural differences play a big role in the story.
In a spiderman defenders crossover fanfiction, the plot might involve a magical threat. Maybe an ancient evil has been awakened in the sewers of New York. Spider - Man stumbles upon it first while swinging around. He tries to fight it but quickly realizes he needs help. The Defenders, who are more in tune with the street - level, darker side of the city, are also aware of the danger. They all come together. Spider - Man can use his scientific knowledge to help figure out the origin of the threat, while the Defenders use their combat skills and street smarts to take on the minions of the ancient evil.
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.
The Green Goblin could be a great main villain. He could be working on a plan that not only threatens Spider - Man's world but also the areas the Defenders protect. His maniacal nature and flying abilities would pose a great challenge to both Spider - Man's acrobatics and the Defenders' combat skills.
Sure. You can check out Archive of Our Own (AO3). There are a lot of great 'defenders crossover fanfiction' works there. Just search for 'defenders crossover' and you'll find various stories that combine the characters from 'The Defenders' with other universes or characters.
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.
Definitely Daredevil and Black Widow. Daredevil's enhanced senses and martial arts skills combined with Black Widow's espionage expertise and combat abilities would make for an exciting duo. They could work together on a stealth mission, with Daredevil using his senses to detect hidden threats and Black Widow using her gadgets and charm to get them in and out of dangerous situations.
Doctor Strange and Thor would be an interesting pair. Thor with his Asgardian magic and brute strength, and Doctor Strange with his powerful sorcery. They might have some initial clashes over whose magic is more powerful, but then come to respect each other and use their combined powers to defeat a powerful magical enemy.
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.
You could write a fanfiction about the backstories of the Defenders. For example, delving deeper into Matt Murdock's training before he became Daredevil. This would give readers a better understanding of the characters.