A really cool fanfiction could be Steven in a Death Battle against a powerful alien race. His ability to communicate and understand different beings could play a big role. He might try to find a peaceful solution at first, but when pushed, his combat abilities would come into play. It could show how he grows as a character in a high - stress combat situation.
One idea could be Steven vs a powerful magical being from another series. His shield and gem powers would make for an exciting battle.
Maybe Steven in a Death Battle against a character with similar powers but different motives. For example, a character who also has the power to heal but uses it for evil. In the fanfiction, it could explore how Steven's positive nature would be put to the test.
Another possibility is a crossover with "Gravity Falls". Dipper and Mabel could come to Beach City during a mystery involving Gem technology. Their knowledge of the strange and unusual from Gravity Falls could combine with Steven's understanding of Gem lore to solve the mystery.
One idea could be that as a girl, Steven has different relationships with the other Crystal Gems. Maybe she forms a really close sister - like bond with Pearl, who becomes overprotective. Another could be that her fusion forms take on a more feminine aesthetic and she has unique experiences when fusing with the others.
One idea could be that the Crystal Gems find a strange Pokemon in the woods near Beach City. They initially mistake it for a new type of Gem monster. For example, a Pikachu that has an electric aura similar to the energy the Gems use. The Gems then try to figure out how to deal with this 'creature' which leads to humorous and heartwarming interactions as they learn about Pokemon.
Another good opponent could be a character like Naruto from Naruto. Their powers are very different. Naruto has his ninjutsu and chakra, while Steven has his gem abilities. The clash of their ideals and fighting styles would make for a great fanfiction Death Battle.
One idea could be that Lars and Steven go on a space adventure together. They might encounter new gem species and have to work together to solve problems. For example, they could find a planet where the gems are in a civil war, and Lars and Steven use their unique skills to bring peace.
Perhaps in the fanfiction, there is a threat that only the combined forces of the Naruto universe and the Steven Universe can stop. Naruto and his friends team up with Steven, Garnet, Amethyst, and Pearl. They have to learn to work together, with Naruto's combat skills and the Gems' unique abilities being essential in defeating the common enemy.
Another idea is a human OC who is a detective in Beach City. They start to notice strange happenings that are related to Gem activity. As they dig deeper, they get involved with the Crystal Gems and become an ally. They could use their detective skills to help the Gems solve mysteries, like finding missing Gem shards or uncovering the secrets of an ancient Gem structure.
A more emotional plot could be an OC who was once part of the Homeworld gems but defected. They have a lot of guilt and self - doubt because of their past actions. In the fanfiction, they try to make amends by helping the Crystal Gems. But they constantly fear that their old allies will come after them. This internal struggle and their journey of redemption could be a really interesting story.
Maybe a story where Steven, being his empathetic self, tries to understand the complex and often morally grey world of JoJo. He could meet Jotaro and learn about Stands. In turn, Jotaro could be intrigued by the Gems and their unique way of life. This could lead to a series of adventures as they try to find common ground between their two very different worlds.
One idea could be that Ichigo from Bleach ends up in the Steven Universe world. He might be confused by the Gems' powers at first. Then he could learn from them and they from him. For example, he could teach the Crystal Gems about his Hollow powers, and in return, they could show him how to use Gem magic in a new way.