If it was a crossover with "She - Ra and the Princesses of Power", Adora could be a main character. Her strong leadership and the power of She - Ra would be an interesting contrast to Steven's more laid - back leadership style. Together, they could fight against a common enemy that threatens both their worlds.
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.
A time travel fanfiction could involve a character other than Steven, like Connie. She could travel back in time to meet Rose Quartz. This would be an interesting encounter as Connie could ask Rose about her past, her relationship with Greg, and her hopes for Steven. It could also create a situation where Connie has to make sure she doesn't accidentally change the past in a way that would harm the present.
In 'Steven Universe Watch the Future fanfiction', Steven is bound to be a central figure. He's the protagonist of the original series, after all. Alongside him, the Crystal Gems - Garnet, Amethyst, and Pearl - are very likely to be main characters. They've been with Steven through thick and thin. Additionally, characters like Peridot, Lapis Lazuli, and Bismuth, who have become important parts of Steven's life in the later parts of the series, could also be main characters. Connie, with her connection to Steven and her own growth in the story, is also a strong candidate for a main character.
One common theme is self - discovery. When a character travels through time, they often learn more about themselves. For example, Steven might travel back to a time when he was younger and realize how much he has grown. Another theme is fixing past mistakes. Maybe a character goes back to prevent a certain event that had negative consequences in the future.
In the context of the show's universe, it's a bit of a strange concept. Since fanfiction exists in our real - world fandom and not in the show's world directly. But if we were to imagine a meta - situation where he could access it, I suppose it could be possible. However, it would be a very surreal situation for him.
Peridot is quite popular too. Her unique personality, especially her growth from a somewhat antagonist to a more friendly character, makes for great storytelling in crossovers. In a crossover with 'Ben 10', Peridot's technological knowledge could be paired with Ben's Omnitrix - related adventures.
Well, it could be really cool. Maybe the Gems would be intrigued by the technology of the Omnitrix. Steven might try to understand Ben's powers better and see if there are similarities between his gem powers and Ben's alien forms. And Ben could learn about the Gem culture. They could also exchange stories about their adventures and how they protect their respective universes.
To write a good Steven Universe FNAF crossover fanfiction, you need to consider the tone. Since Steven Universe is more light - hearted in general and FNAF is horror - based, find a balance. You could start with the Steven Universe characters in their normal setting and then gradually introduce the FNAF elements. Let the characters react realistically to the new and scary situation. For instance, Amethyst might initially be curious about the animatronics but then get scared when they start acting strangely.
It could be really interesting. Maybe the gems from Steven Universe end up in the spooky world of FNAF. They could use their powers to fight off the animatronics. For example, Garnet's strength could be used to hold the animatronics back while Pearl uses her spear to disable them.