Well, you could have an OC who is an alien with special powers that are different from the normal Semblances in RWBY. For example, they can control gravity in a way that gives them an edge in combat. Or an OC that is a diplomat from a space - faring civilization trying to make peace between different kingdoms on Remnant. Also, an OC who was stranded on Remnant after their spaceship crashed and has to adapt to the world while also trying to find a way back home.
An interesting concept for RWBY space OC fanfiction is an OC that is a member of a space - based resistance against an evil empire that has its eyes on Remnant's resources. This OC could bring new fighting styles and technology to the RWBY characters. Another idea is an OC who is a space explorer and discovers a new type of Dust in outer space that has incredible powers. And then there's the idea of an OC who is a time - traveler from the future of RWBY's universe who has to use space - related technology to fix a major problem in the past.
An OC in RWBY fanfiction could be a character who is a master of stealth. They could use their skills to gather intelligence for the teams at Beacon Academy. Their backstory could involve growing up in a family of thieves, but they decided to use their abilities for good. They might have a Semblance that allows them to become invisible for short periods of time, but it has a drawback like draining their Aura quickly. This OC could interact with characters like Blake, who also has some stealth - like abilities, and they could learn from each other.
For RWBY OC fanfiction, you could make an OC who has the power to manipulate time on a small scale. This would be a really cool ability in battles, being able to slow down opponents or speed up oneself. Another idea is to have an OC that is a faunus with a very rare animal trait, like a phoenix - like faunus. Their story could revolve around hiding their true nature because of discrimination. And an OC that is a transfer student from a far - off land with a completely different fighting style compared to the students at Beacon Academy would also make for an exciting story.
An OC could be a stowaway on the ship in the 'lost in space' universe. This stowaway has a mysterious past and special skills, like being able to communicate with an alien species that the main crew doesn't know about. Their presence could cause both chaos and new opportunities for the crew as they try to find their way home.
One idea could be creating an OC who is a skilled mechanic from a far - off planet. They could join the crew of the Galaxy One and use their unique mechanical skills to fix and upgrade the ship in ways that the others can't. Maybe they have a special tool that was passed down in their family for generations.
Well, you could have an OC mercenary who has a special bond with a Grimm. Instead of fighting Grimm all the time, this OC has found a way to communicate and even control a certain type of Grimm. Maybe it's a small, less - dangerous Grimm at first, but as the story progresses, they learn to handle more powerful ones. Or an OC who is a technologically - advanced mercenary, using high - tech gadgets and weapons that no one in Remnant has seen before. Also, an OC with a tragic past as a mercenary, like they were once part of a group that was betrayed, and now they're on a quest for revenge while still taking on mercenary jobs.
You could make your OC have a special power related to speed. They could be faster than any of the RWBY characters, which would make for some exciting fight scenes. Their motivation could be to avenge their family who were killed by Grimm. In the fanfiction, they start out as a loner but gradually form bonds with the main RWBY characters as they realize they have a common enemy.
One idea could be an OC who is a healer in the hospital and has to deal with the unique injuries of RWBY characters. For example, they might struggle to heal a Grimm - inflicted wound that has some sort of dark magic attached to it. Another idea is an OC that discovers a new type of Dust - based medicine in the hospital, which could revolutionize the way injuries are treated in the RWBY world. And also, an OC who is a patient in the hospital and has amnesia, and throughout the fanfiction, they slowly regain their memories and realize they have a special connection to the RWBY universe.
One idea could be to cross your original character (OC) with the world of RWBY and have them be a transfer student at Beacon Academy. They could bring unique skills or powers from their own world, like a magic system that RWBY doesn't have. Another idea is to have the OC be from a different time period and get thrown into the RWBY universe, causing all sorts of chaos and new storylines. For example, an OC from a steampunk world with advanced machinery could introduce new tech to the RWBY characters.
Another interesting concept is an OC that is part of a secret society in the RWBY Cold world. This society could be dedicated to protecting ancient ice - related artifacts. The OC would have to navigate the politics of the RWBY universe while also keeping their society's secrets. They might form alliances with the main RWBY characters to fend off villains who want to use the artifacts for evil.
One idea could be creating a robot OC that has the ability to transform into different forms inspired by the various Grimm in RWBY. For example, it could shift into a Beowolf - like form for combat situations. Another idea is a robot OC designed by a villainous character in the RWBY universe but ends up having a change of heart and joins the heroes. You could also have a robot OC that has a unique Dust - based power system, which is different from the normal ways Dust is used in RWBY.