There could be a fanfiction where Superman is accidentally transported to the Avengers' universe. At first, there might be some mistrust from the Avengers as they don't know him. But as they see his heroic deeds, they start to work together. For example, Superman could help Captain America in a fight against a super - powered foe by using his speed to quickly take out the enemy's support forces, while Captain America leads the main assault.
Well, in some fanfiction, Superman might teach the Avengers about his Kryptonian technology. This could lead to new inventions within the Avengers' world. Also, a crossover could involve Superman's nemesis, Lex Luthor, teaming up with one of the Avengers' villains, like Ultron. It would create a really complex and exciting plot with two brilliant minds causing chaos for both Superman and the Avengers.
One interesting crossover is when Superman's code of ethics clashes with the Avengers' more flexible approach. It creates a lot of drama and character development. For instance, in some stories, Superman has to decide whether to follow the Avengers' plan which might involve a bit of collateral damage, something he's not used to in his own world.
One interesting crossover could be Naruto learning about Superman's powers. Maybe Naruto is intrigued by Superman's ability to fly and his super strength. He might try to incorporate some of those elements into his own jutsu training. For example, he could study how Superman uses his strength in combat and adapt it to his taijutsu.
In some fanfictions, there might be a plot where the Avengers mistake Ben for a threat at first. But then they realize his good intentions when he helps them fight a common enemy. This adds an element of misunderstanding and then friendship, which is quite common and engaging in crossovers.
One interesting crossover could be Godzilla teaming up with the Hulk. They are both powerful and somewhat uncontrollable forces. Imagine them fighting side by side against a common enemy, like an alien invasion that threatens both Earth and the Avengers' world. The contrast between Godzilla's raw, brute strength and the Hulk's gamma - powered rage would make for an exciting story.
In some fanfictions, the technology in Ben 10's world might be combined with Stark's technology in the Avengers. Maybe Ben's Omnitrix could be enhanced by Stark's knowledge, creating a more powerful device that helps them in a battle against an alien invasion. This would involve a lot of scientific discussions and teamwork between Ben and the Avengers, especially Tony Stark.
A crossover could involve the Homunculi from Fullmetal Alchemist being a threat that the Avengers have to face. The Avengers would be challenged by their unique abilities and the complex nature of the Homunculi. The alchemists might then team up with the Avengers, sharing their knowledge of the Philosopher's Stone and its power, which could be crucial in fighting the Homunculi.
One interesting crossover could be a story where the Avengers are facing a threat that has a supernatural origin, and they seek the help of the TFW. Maybe the Avengers are dealing with a horde of demons that they can't defeat with their usual technology and powers, and Sam, Dean, and Castiel come to the rescue with their knowledge of the supernatural world.
In some fanfictions, Sailor Moon herself could team up with Captain America. Their leadership qualities would blend well. Sailor Moon's positive energy and ability to inspire could be a great boost to the Avengers' morale. Meanwhile, Captain America's combat skills and strategic thinking would be useful in the battles they face together in this crossover universe.
One interesting crossover could be Danny using his ghost powers to help the Avengers in a battle against a powerful, otherworldly threat. For example, his intangibility might be useful for sneaking past enemy defenses.
In Phineas and Ferb Avengers fanfiction, a common crossover idea is the Avengers being called to the Phineas and Ferb universe because of some sort of energy anomaly. This anomaly could be caused by one of Doofenshmirtz's inventions gone wrong. Hawkeye might find himself in a strange situation where he has to use his archery skills in the wacky world of Phineas and Ferb. Hulk could be both a source of amazement and a bit of a problem as his large size might accidentally destroy some of the boys' projects. The crossovers often play with the differences in the two universes' rules and characters' personalities to create unique and fun stories.