Well, a great crossover concept could be the heroes from One Punch Man training with the Crystal Gems. Imagine Genos learning about gem magic from Garnet or Pearl. It would be a whole new experience for him. Also, the idea of Steven trying to understand Saitama's non -chalant attitude towards his incredible power could be really fun to write about. Maybe Steven could even try to find a way to make Saitama care more about his reputation as a hero, like he does for himself in his own universe.
Steven's kind - hearted and empathetic nature could have an impact on Saitama. Saitama might start to show more emotion or care for others on a deeper level. On the other hand, Saitama's laid - back and confident attitude could rub off on Steven, making him more self - assured in dangerous situations. For example, when facing a tough gem - related problem, Steven could be like Saitama and just say 'It's no big deal' and handle it calmly.
It expands the universe by exploring the backstories of the characters more deeply. For example, we might learn more about the childhood of characters like King. It also creates new scenarios for the heroes to face, like battles in different dimensions. This gives more depth to the world of One Punch Man.
Deku would likely become more confident in his abilities. In the One Punch Man universe, he'd see Saitama's nonchalant attitude towards fighting and might start to adopt a similar mindset. He'd also have to adapt to the different types of villains. Since One Punch Man villains are often very strange and powerful in unique ways, Deku would need to be more creative with his strategies.
A common plotline involves Spinel struggling to fit in on Earth, and Steven being her guide. He shows her the beauty of Earth and its inhabitants. This can lead to a deepening of their relationship as Spinel starts to trust and rely on Steven more. Another plotline could be Spinel having nightmares about her past, and Steven being there to comfort her, which further cements their bond.
The relationship in this fanfiction often explores themes of forgiveness and growth. Steven, with his nature of seeing the good in others, tries to help Spinel move past her pain. Spinel, who has been through so much, slowly starts to open up to Steven. It's a relationship that can be seen as a journey of two broken souls finding comfort in one another.
Si could be introduced as a new trainee at the Hero Association. He might start off as an underdog, constantly underestimated by the other heroes. But as he faces various challenges and battles, he gradually reveals his hidden potential. For example, in a battle against a powerful monster, he uses his unique skills, like being able to read the monster's movements really well, to gain an advantage.
Adventure is also a common theme. They go on different quests or face various threats together. This could be something like exploring an ancient gem ruin on Earth or fighting off a new enemy. The theme of acceptance is important too. Accepting each other's flaws and differences. Amethyst accepting Steven's sometimes naive view of the world, and Steven accepting Amethyst's wilder side.
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.
You can try Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Steven Universe. Just use the search bar and type in 'Spinel x Steven' to find relevant stories.