A plot could involve a half - gem OC who is half Emerald and half Peridot. The OC is discovered by a group of rogue gems who want to use her unique powers for their own gain. She has to escape from them and find her way to the Crystal Gems. Along the way, she discovers that she has the ability to create energy shields like an Emerald and hack into technology like a Peridot. Another idea is a half - gem OC half Topaz and half Bismuth. She is born during a time of peace but when a new war breaks out between Homeworld and the Crystal Gems, she has to decide which side to fight for, as she has qualities from both sides.
One interesting plot for a half - gem OC fanfiction could be centered around an OC who is half Garnet and half Jasper. This OC would be incredibly powerful but also conflicted. Since Garnet is a fusion and Jasper is a Homeworld gem, the OC would be constantly torn between the values of the Crystal Gems and the strict hierarchy of Homeworld. Maybe she is initially on Homeworld's side but then has a change of heart after meeting Steven and seeing the love and acceptance within the Crystal Gems. Her journey could be about finding her true self and where she belongs in this complex universe of gems.
One idea could be a gem oc who was created with a unique power that the other gems don't understand. For example, the power to communicate with non - sentient objects. This could lead to all sorts of adventures as the oc tries to figure out how to use this power and the other gems try to figure out if it's a threat or a valuable asset.
One idea could be an OC named Ruby - Quartz. She is half Ruby and half Quartz. Her story could start with her feeling out of place among the full - gem characters. She has the fiery passion of a Ruby but the calm stability of a Quartz. As she tries to find her identity, she meets Steven and the Crystal Gems. They help her understand that being a half - gem doesn't make her any less special. She then uses her unique combination of powers to help save the day during a battle against a new Homeworld threat.
A more emotional plot could be an OC who was once part of the Homeworld gems but defected. They have a lot of guilt and self - doubt because of their past actions. In the fanfiction, they try to make amends by helping the Crystal Gems. But they constantly fear that their old allies will come after them. This internal struggle and their journey of redemption could be a really interesting story.
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.
One interesting plot twist could be that a previously thought - to - be - evil gem actually has a noble reason for their actions. For example, a gem who was fighting against the Crystal Gems might be trying to protect a hidden colony of gems that would be in danger if the war ended a certain way.
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 common theme is self - discovery. Gem OCs often have to figure out who they are in the context of the Steven Universe world. For example, they might be different from the known gem types and have to learn about their own powers and purpose.
Another idea is a human OC who is a detective in Beach City. They start to notice strange happenings that are related to Gem activity. As they dig deeper, they get involved with the Crystal Gems and become an ally. They could use their detective skills to help the Gems solve mysteries, like finding missing Gem shards or uncovering the secrets of an ancient Gem structure.
Maybe the giant OC is a guardian of a long - lost power. The story could revolve around Steven and the gang trying to figure out how to communicate with this giant, who seems to be protecting something important. It could lead to adventures where they have to learn the giant's language and culture, which is very different from their own.
One idea could be an OC Diamond who has different powers compared to the known Diamonds. For example, the ability to control time in a limited way. This could create interesting storylines where the OC Diamond tries to fix past mistakes within the Steven Universe world.