A possible fanfiction could involve Steven having the ability to communicate with Pokemon. He uses this power to bring together the Pokemon world and the world of Steven Universe. He might organize a big event where Pokemon trainers and the Crystal Gems meet and exchange their skills and knowledge. This could lead to new friendships and alliances, like a trainer teaching Pearl how to be more strategic in battles using Pokemon battle tactics.
Perhaps there could be a story where a powerful and rare Pokemon is drawn to the power of the Gems. The Pokemon has some sort of problem that the Gems can help with, like it's being chased by an evil organization in the Pokemon world. In return, the Pokemon offers its unique abilities to help the Gems protect Beach City from a new and powerful enemy that neither could face alone.
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.
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.
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 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.
Another idea is that there is a fusion between a Pokemon and a Gem. This fusion could have a very distinct appearance and abilities. It could struggle to find its place in both the Pokemon world and the Steven Universe world. The story could then revolve around how it tries to fit in and how the characters from both universes react to this new and strange being.
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.
One idea could be an OC Diamond who has different powers compared to the known Diamonds. For example, the ability to control time in a limited way. This could create interesting storylines where the OC Diamond tries to fix past mistakes within the Steven Universe world.
One idea could be a crossover with "Adventure Time". Finn and Jake could visit Beach City and interact with the Crystal Gems. They could go on a mission together to stop a new threat that combines the magic of both universes.
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.