A possible plot is that Naruto and Steven are summoned to a world where a prophecy states they will form a harem to bring balance. Each member of the harem has a special role or power related to either Naruto's world of ninjas or Steven's gem world. As they interact, there are comical misunderstandings due to their different backgrounds, but they gradually form deep bonds and work towards fulfilling the prophecy.
Perhaps Steven finds himself in a situation where he has to protect a number of characters, and in the process, they all fall for him. Like he could be on a mission to save some of the corrupted Gems, and as he heals them, they see his pure intentions and are drawn to him. This could lead to a harem where they all vie for his attention in different ways, some being more shy and others more bold in their affections.
Well, the OC could be a human - gem hybrid. This would make them an outcast in both human and gem society at first. But as the story progresses, some of the main characters like Pearl, Amethyst, and Peridot start to form a bond with the OC. Maybe they are all intrigued by the OC's dual nature and the way they navigate between the two worlds. This could lead to a harem - like situation where they all have different relationships with the OC, some more platonic, some with a hint of romance.
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.
One idea could be that Naruto and his friends end up in the world of Steven Universe. They could team up with the Crystal Gems to fight a new common enemy that has the power - stealing abilities like Orochimaru and the ability to corrupt gems like the Diamonds' powers. Naruto's jutsu could be a new and strange concept for the Gems, and Steven could try to understand Naruto's ninja way.
One idea could be that Steven tries to understand Blue Diamond's past traumas and helps her heal. Maybe he uses his unique powers of empathy to break through her cold exterior.
One idea could be that the reader is a new gem on Earth and Spinel is initially suspicious but then they become friends through exploring human culture together. For example, they could go to a carnival and Spinel discovers the joy of cotton candy.
One idea could be that the OC is a new gem who arrives on Earth and Lapis is initially suspicious but gradually warms up to them as they show their kind nature. For example, the OC might help Lapis with her water powers, like finding new ways to control and shape water in a more creative way.
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.
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.