Perhaps she's a time - traveler from a future where the events in Steven's world have led to some big changes. She could be on a mission to prevent a certain catastrophe from happening, and her presence in Steven's time is a crucial part of that plan. She might have knowledge of future events and technologies that seem strange to Steven and the others.
She could be an ordinary human who has a special gift for sensing gem energy. This gift has led her to Beach City, where the gem activity is high. Her character arc could be about learning to control and use this gift, with Steven and the Crystal Gems helping her along the way. And as she spends more time with them, she becomes an important part of their world.
A possible element in such a fanfiction could be a hidden gemstone in the mystery girl's navel. It could be a new type of gem that has powers unknown to the Crystal Gems. The story could revolve around the discovery of these powers and how they affect the balance in the Steven Universe world.
In a small corner of Beach City, the Mystery Girl set up a little booth selling handmade jewelry. Steven was curious and went to check it out. He saw that the jewelry was made with some unique gemstones that he had never seen before. The Mystery Girl told him that she had collected these stones from all over the world during her travels. As they talked, Steven noticed that she had a small tattoo on her wrist that looked like a gem symbol. He asked her about it, but she just smiled and changed the subject. This made Steven even more determined to uncover the secrets of this fascinating girl.
I'm not sure if there are a lot of specific fanfictions just about the mystery girl's navel in Steven Universe. However, it could be an interesting concept for a story. Maybe the navel could be a unique feature that has some kind of magical or symbolic meaning in the fanfiction. For example, it could be the source of her power or a connection to an ancient secret.
One possible plot could be that Pearl is initially very curious about the Mystery Girl. She might observe her from a distance, trying to figure out her habits and what makes her so mysterious. Then, they could accidentally meet, and at first, Pearl is a bit clumsy in her interaction due to her own insecurities. But as time goes on, they start to bond over shared interests like music or art. The Mystery Girl could introduce Pearl to new aspects of human culture that Pearl has not fully experienced yet.
If Steven turns into a girl in fanfiction, it could change the way he interacts with the other characters. For example, his relationship with Connie might become more complex, as they could explore a different kind of friendship or even a romantic relationship. Also, the way the Crystal Gems protect him might change, as they may see him as more vulnerable in his new form.
One idea could be that Steven turns into a girl due to a magical mishap. Maybe he was trying to help a gem with a spell and it backfired. As a girl, he could struggle with how others perceive him differently, but also discover new aspects of his personality. He might have to learn new ways to fight and use his powers, adding an interesting twist to the story.
A simple plot could be Baby Steven getting lost in the woods near the Crystal Gem base. He encounters some friendly forest creatures who help him find his way back. As he wanders, he discovers new magical elements in the forest that the Gems didn't even know about.
One idea could be that Pearl and Mystery Girl team up to solve a mystery on Earth that involves Gem technology. Maybe they find an ancient Gem artifact that is causing strange phenomena, and they have to figure out how to stop it together.
In the context of the show, since Sapphire is in a relationship with Ruby, if we consider the concept of a 'father' in a non - traditional sense, Ruby could be seen as the co - parent. Gems don't reproduce in the same way as humans, but their relationship could be the equivalent of a parental unit in this fanfiction.
Besides Steven, the main characters could include the villains he's faced in the past. Now that he's a girl, they might underestimate him or treat him differently. His father Greg could also play a big role. Greg might be a bit shocked at first but then would be super supportive, trying to understand what his 'new' child is going through. And there could be new characters introduced, like other girls in the town who become friends with Steven and help him adapt to the female world.