A third plot idea is that the Faunus OC is a healer. Weiss gets injured during a battle and the Faunus OC is the only one around who can help. Weiss is initially hesitant to accept help from a Faunus but has no choice. As the OC heals her, they start to talk and find out they have a lot in common, like their love for their families and their determination to protect Remnant. This starts a friendship that could potentially turn into something more.
One plot idea could be that the Faunus OC is initially misunderstood by Weiss. For example, the OC might be a cat Faunus and Weiss has some preconceived notions about cat Faunus being sly. But as they are forced to work together on a mission in Beacon, they start to see each other's true qualities. The OC could show great courage and loyalty, which makes Weiss question her own biases.
Maybe a fanfic where Weiss discovers a hidden Faunus community while exploring. She gets separated from her team and has to rely on the Faunus there to survive. This experience makes her re - evaluate her thoughts on Faunus. The Faunus in the community could have unique cultures and abilities that Weiss has never seen before, and she becomes fascinated by them.
I really like 'Weiss and the Faunus Bond'. In this fanfiction, Weiss discovers the unique aspects of Faunus culture through her interactions. It shows her growth as a character as she overcomes her initial prejudices. It also has some great action scenes intertwined with the emotional development.
In some fanfictions, the idea of Weiss using her family's influence to advocate for Faunus rights is quite unique. For example, in 'Weiss's Advocacy', she starts a movement within her family's company to promote equality for Faunus. It's an interesting concept as it goes against the typical view of the Schnee family's relationship with the Faunus.
A plot could be that Weiss meets a Faunus with a special power that she's never seen before. They start off on the wrong foot, but as they are forced to team up against a common enemy, like a new type of Grimm. As they fight side by side, Weiss realizes that the Faunus is not what she expected and they develop a mutual respect for each other.
To develop their relationship realistically, start with conflict. Since Weiss has some prejudices about Faunus, there should be initial tension. For example, the Faunus OC could be in a group that Weiss believes is causing trouble at Beacon. This will make Weiss avoid the OC at first. Then, through a series of events like group projects or battles where they have to rely on each other, they start to break down those barriers. Small interactions like sharing a meal in the cafeteria or helping each other study can also slowly build their relationship.
One idea could be a story where Ruby, as a Faunus, has to hide her identity in a world that's still somewhat prejudiced against Faunus. Weiss discovers her secret and at first is conflicted because of her family's traditional views. But as they go on adventures together, Weiss realizes that Ruby is still the same brave and kind person she always knew, and they form an even deeper bond.
One idea for character development could be exploring the Crow Faunus' backstory. Maybe they come from a family of outcasts within the Faunus community due to their crow features. This could lead to a sense of isolation and a drive to prove themselves. Another aspect could be their relationship with other characters in the RWBY universe. They might form an unlikely alliance with a human character who initially distrusts Faunus, but over time they learn to trust and rely on each other.
First, do your research on the RWBY universe, especially about Weiss and the Faunus lore. Know their characteristics, powers, and the existing relationships between humans and Faunus. Then, come up with an interesting plot. It could be something like Weiss having to protect a Faunus friend from a group of anti - Faunus hunters. Make your characters believable. Give them flaws and growth. For Weiss, her pride as a Schnee could be a flaw that she has to overcome when dealing with Faunus. Also, use vivid descriptions of the RWBY world, like the unique landscapes and the different Dust - based technologies.
A male Faunus with bat wings would be cool. He could be nocturnal and have enhanced senses at night. His story in the fanfiction could start with him being a bit of an outcast because of his appearance. But then he discovers a hidden power within himself that could potentially save the world of RWBY. He could form a special bond with one of the main characters like Ruby and they could help each other grow.
One theme could be acceptance. Weiss, coming from a more traditional and perhaps elitist background, has to learn to accept Ruby as a Faunus. Ruby, on the other hand, has to accept that Weiss may not immediately understand her situation but is trying. Another theme could be friendship. Despite their differences, they form a strong bond through shared experiences at Beacon Academy.