Perhaps there could be a story where the Infinity Stones in the Avengers universe are actually related to the Gems in Steven Universe. The power of the Infinity Stones starts to affect the balance in the Steven Universe world, and the Avengers come to help the Crystal Gems figure out how to stop the chaos. This would bring together the different power systems and mythologies of both universes in a really exciting way.
A fanfiction could involve a crossover where the Avengers are shrunk down to the size of the Gems in Steven Universe. They have to adapt to this new world and its rules. For example, they might have to learn how to use Gem weapons and powers. In return, they could teach the Gems about human combat techniques that are different from what the Gems are used to. This would create a lot of humorous and action - packed situations.
One idea could be that Iron Man somehow ends up in the Steven Universe world. Maybe his technology malfunctions during an experiment and transports him there. He could then use his tech knowledge to help the Crystal Gems with their battles against the Homeworld gems. For example, he could create new types of shields or weapons using the unique materials in the Steven Universe world.
One idea could be a crossover where Ladybug and Cat Noir end up in the world of Steven Universe. They could team up with Steven and the Crystal Gems to fight a common enemy that has the power to disrupt both their universes.
One idea could be a crossover where Naruto's world is invaded by the Gem Homeworld from Steven Universe, and the Juubi's power somehow gets involved in the conflict. For example, Naruto and his friends might have to team up with Steven and the Crystal Gems to fight off the combined threats.
A great fanfiction could be about Steven and Connie swapping bodies. This would lead to a lot of humorous moments as they try to figure out how to act like the other. Connie would struggle with Steven's powers, and Steven would have to deal with Connie's more studious and disciplined life. It could also be a chance for them to understand each other on a deeper level.
Perhaps Goku lands in New York City and his power level is sensed by the Avengers. Iron Man is initially skeptical but Captain America is more open - minded. They test Goku's abilities and find out he can move at incredible speeds and has near - invincible defenses. So they ask him to train the Avengers in new combat techniques, which leads to some humorous and action - packed training sessions.
Another possibility is that the Avengers end up in the OHSHC world. The Avengers would be really out of place in that high - school - like setting. Thor might be really confused by all the tea parties and formalities, while Iron Man would probably try to find a way to use the technology there, which is much less advanced than his own. Black Widow could blend in a bit better, using her espionage skills to navigate the social hierarchies of the OHSHC club.
A fanfiction could also be based on the Avengers trying to find a way to reverse the Titan transformation. They might see the Titans as victims of some sort of dark magic or science gone wrong. Bruce Banner, with his scientific knowledge, teams up with Armin from Attack on Titan. Armin provides all the information he has about the Titans while Banner tries to come up with a solution. Meanwhile, the rest of the Avengers are fending off any Titans that come their way.
A possible fanfiction could involve a crossover where some of the Avengers are sent to Camp Half - Blood. Imagine Tony Stark trying to figure out how the magic at the camp works while also being highly amused by the whole concept of demigods. Steve Rogers could bond with some of the more heroic demigods over their shared sense of duty. Meanwhile, Percy and his friends have to keep an eye on the Avengers to make sure they don't accidentally disrupt the delicate balance of the camp. And then, they all have to face a threat that targets both the Avengers and the demigods, like a group of villains who want to steal the powers of both.
Maybe in the fanfiction, the Avengers need someone with John Wick's particular set of skills for a stealth mission. John Wick initially refuses but is later convinced when he realizes the threat is global. During the mission, they learn from each other's combat styles and strategies, and John Wick's no - nonsense attitude impresses the Avengers.
One idea could be that Naruto is somehow transported to the Marvel universe where the Avengers are. He could use his ninja skills, like his shadow clone jutsu, to confuse the villains that the Avengers are facing. Maybe he teams up with Iron Man and they combine their tech and ninjutsu to create some really cool weapons or defenses.