Maybe the OC is a Grimm researcher who is specifically interested in Dragon Grimm. They go on an expedition to find a rare type of Dragon Grimm. Along the way, they encounter a group of Huntsmen and Huntresses who initially suspect the OC of being a threat. But as they face challenges together, they start to trust each other. The OC's knowledge about Dragon Grimm helps them all survive various attacks from other Grimm.
Consider the origin of the OC Dragon Grimm. It could be a result of some ancient magic gone wrong. For its design, add some mechanical - like parts to its body if you want a more steampunk feel. In terms of abilities, it could have the power to teleport short distances. And for its personality, make it more intelligent and cunning. It could use its intelligence to outwit its opponents, especially the Huntsmen and Huntresses in the RWBY universe.
Well, it could be that Naruto somehow gets transported to the RWBY universe and has to deal with the Grimm. Maybe he uses his ninja skills to fight them off. For example, his shadow clone jutsu could be used to confuse the Grimm while he attacks from another angle.
One concept could be that the OC is a half - Grimm who struggles with their identity. They might be shunned by both humans and Grimm at first. But as the story progresses, they use their unique abilities, like being able to sense Grimm better than humans and having some Grimm - like strength, to help the RWBY team in their battles. For example, they could be able to sneak into Grimm nests without being detected as easily as a normal human.
One possible plot could be a Dragonborn OC who arrives in the RWBY world unexpectedly. This OC could have unique dragon powers that both attract and frighten the RWBY characters. They might initially be seen as a threat but then prove to be an ally in the fight against Grimm, using their dragon breath and enhanced strength to turn the tide in battles.
One possible plot could be Ruby initially trying to hide her Grimm part from her friends. As she struggles with newfound abilities and instincts, she has to constantly fight the urge to give in to the Grimm side. Meanwhile, a powerful enemy emerges that she might be uniquely suited to defeat because of her Grimm part, but she has to first come to terms with it herself.
Well, it could be about Ruby struggling to hide her true nature as a Grimm Faunus while still being a Huntress. Maybe she has to deal with the prejudice from others who fear Grimm Faunus.
Perhaps Harry Potter finds himself in the world of RWBY and has to deal with the Grimm. He could use his magic to fight against the Grimm while also learning about the unique powers and technology in the RWBY universe.
One common theme is the struggle for acceptance. Since the OC is often different due to their Grimm - related aspects, they have to fight to be accepted by the RWBY characters. Another is power and its control. Dealing with Grimm - based powers can be a double - edged sword.
Well, a possible plot in 'rwby controls grimm fanfiction' is that the whole team RWBY starts to experiment with Ruby's new - found ability to control Grimm. They could face moral dilemmas, like whether it's right to use the Grimm as weapons. Maybe they try to use controlled Grimm to gather intelligence about Salem's plans. Or, they could accidentally create a new type of Grimm - hybrid when Ruby's control goes a bit wrong during an experiment.
Perhaps a half - Grimm character is hunted by both Grimm and humans. The Grimm see it as an abomination, and the humans fear its power. The story could follow their journey to find a place where they can be accepted. They might encounter various characters from RWBY along the way, and their interactions could lead to different moral dilemmas.