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 interesting crossover could be Naruto characters in the world of Steven Universe. For example, Naruto's ability to use chakra could be a unique power in the SU universe. Maybe he could use his Shadow Clone Jutsu to help out in a gem battle. Another could be Steven trying to understand Naruto's ninja way and learning some of his moves to defend Beach City in a new way.
One interesting crossover could be the use of magic in the Steven Universe world. For example, Steven might learn some basic spells from the Harry Potter universe and use them in his own world to solve problems in a new way. Maybe he could use a levitation spell to move the big rocks around on the beach more easily.
In some fanfics, there could be a situation where Steven visits the Naruto world. He might be really confused at first by all the ninja stuff. But then he could start to use his Gem powers in creative ways to help out. He could heal injured ninjas with his healing spit, and the ninjas would be amazed at this strange power from another world.
In Steven Universe and Dragon Ball Z fanfiction, there could be epic battles. Picture Steven and the Crystal Gems teaming up with Goku and his friends against a common enemy. The different fighting styles would make for an exciting story. Also, the concept of fusion in both series could be explored further. What if a character from Steven Universe fused with a Dragon Ball Z character? It would create a whole new and powerful being with a blend of their unique traits.
One interesting crossover could be the characters from Gravity Falls and Steven Universe teaming up to solve a mystery that involves both the Gem world and the strange happenings in Gravity Falls. For example, Dipper and Steven might combine their knowledge of the supernatural to figure out a new threat.
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.
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.
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.
Maybe the giant OC is a guardian of a long - lost power. The story could revolve around Steven and the gang trying to figure out how to communicate with this giant, who seems to be protecting something important. It could lead to adventures where they have to learn the giant's language and culture, which is very different from their own.