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.
A fanfiction could also be about a cross - over where the Infinity Stones from the Marvel Universe are somehow related to the Gemstones in Steven Universe. Iron Man, who has experience with the Infinity Stones, teams up with Steven and the Crystal Gems to protect both universes from a common threat that wants to use the combined power of the stones and gemstones for evil purposes.
Another concept is that Steven, with his healing powers, is brought to the Marvel Universe where he meets Iron Man. Iron Man is initially skeptical of Steven's powers but then realizes their potential. They could team up to heal some of the damage done by villains in the Marvel Universe. And Steven could also learn a lot about advanced technology from Iron Man.
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 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 that Iron Man creates a special suit inspired by Naruto's ninja abilities. For example, he could develop a suit with the ability to perform shadow clones like Naruto. This would give him an edge in battles as he could send out multiple versions of himself to confuse enemies.
Maybe Harry Potter and his friends visit Stark Tower in the Marvel universe. They are amazed by all the advanced technology there. Iron Man could be intrigued by magic and try to figure out how it works, leading to some really interesting interactions between science and magic.
One idea could be that Iron Man's technology gets mixed up with Justice League's alien tech. Maybe he creates a new suit using some of the Kryptonian crystal technology he discovers while working with Superman. This new suit gives him powers similar to Superman for a short time, but it also has some unexpected glitches that lead to humorous or dangerous situations.
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.
Well, Iron Man would probably be really intrigued by the Gems' abilities. He might try to study them scientifically. Steven, on the other hand, would be in awe of Iron Man's high - tech suit. He'd likely ask a million questions about how it works. And Iron Man might be a bit impatient with Steven's naivete at first, but then grow to like his positive attitude.
One idea could be that Steven turns into a girl due to a magical mishap. Maybe he was trying to help a gem with a spell and it backfired. As a girl, he could struggle with how others perceive him differently, but also discover new aspects of his personality. He might have to learn new ways to fight and use his powers, adding an interesting twist to the story.
Another idea is a crossover where the magic from Harry Potter is introduced to the world of Iron - Blooded Orphans. For example, the Tekkadan group discovers a hidden magical artifact. This artifact could give them an edge in their fight against the Gjallarhorn. Maybe it could be a magical weapon that only works for those with pure hearts, similar to the concept of the Sword in the Stone in the Arthurian legends which are somewhat related to the magical world of Harry Potter.