The Steven Universe comics are canon. They expand on the universe and characters presented in the main series, providing additional depth and background.
Steven Universe has comics for sure. They bring new perspectives and stories related to the franchise. Many fans enjoy them as an extension of the show.
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.
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.
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.