One idea could be an OC who is a Hitman but has a change of heart after encountering the RWBY characters. Maybe they start to see the value in protecting the innocent rather than just taking contracts.
An interesting plot could be an OC Hitman who has a unique semblance that makes them perfect for their job. But when they enter the RWBY universe, their semblance starts to act strangely due to the different nature of the world, and they have to adapt while also dealing with the RWBY characters.
One idea could be an OC who has a special power related to time manipulation in the 'hitman reborn' universe. They could use this power to change the outcome of battles or prevent certain tragedies. Another could be an OC that is a long - lost relative of one of the main characters and has a different view on the mafia world, causing some internal conflicts.
Perhaps the OC's Gamer system is glitched in a way that makes them perceive certain Grimm as video game bosses with specific attack patterns. They have to figure out these patterns to defeat the Grimm more effectively, adding an interesting twist to the battles in RWBY.
Another plot could involve the OC reincarnating into an existing character's body. This would lead to a lot of internal conflict as they try to adjust to the new identity while still having memories of their past self. They could also use their new position to change the course of events in the RWBY universe.
You could write an OC fanfic where the OC is a female assassin. She has a rivalry with Agent 47 at first, thinking he is taking all the good contracts. But as the story progresses, they end up having to team up to take down a powerful criminal organization that is targeting assassins. Her special skill could be hacking into high - security systems to gain access to targets.
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.
One idea could be that the OC reincarnates into the RWBY world with some unique powers from their past life. These powers attract the attention of different characters, gradually forming a harem. For example, their power could be related to some ancient magic that is both feared and desired in the RWBY world.
The gamer OC could be from our world and is somehow sucked into the RWBY world. In this new world, he realizes that his knowledge of games gives him an edge. He starts to form a harem as he helps the RWBY characters in different ways, like providing strategic advice during battles. For instance, he might suggest new ways to fight the Grimm based on his gaming experience. The relationships in the harem could develop as they train together and the OC shows his determination and kindness. Some characters might be initially skeptical of him but gradually warm up due to his positive influence.
Maybe the OC has a special power that only Penny can understand or help control. For example, if the OC can manipulate time in small bursts, Penny's robotic nature might be able to analyze it and they could use this power to save the day during a Grimm attack.
Well, the OC in the 'rwby rinnegon oc fanfic' could be an outcast from their own village due to their connection with the rinnegon. They come to Vale seeking a new start. Along the way, they meet the RWBY team. Their story could revolve around trying to find a way to control their rinnegon - related powers which are unstable at first. As they train with the RWBY girls, they discover new aspects of their powers and how they can be used for good. For instance, they might be able to create portals using their powers, but it takes a lot of practice and focus to do it accurately.