In terms of the overall story, Weiss being a secret Faunus could open up many new plotlines. For example, there could be a group within the Faunus community who are skeptical of Weiss because of her family's past actions. She would have to prove herself to them. There could also be a sub - plot where Weiss tries to find out more about her Faunus heritage, leading to adventures and discoveries. And her combat style might even change as she embraces her Faunus abilities, if any. This would make her a more complex and interesting character in the RWBY fanfiction world.
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.
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.
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.
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.
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.
She would definitely have an impact on Weiss's relationship with Ruby. Ruby would be curious about the new addition and might try to befriend the twin sister. This could cause some jealousy in Weiss at first as she feels Ruby's attention is being divided. But over time, they would all form a stronger bond. With Blake and Yang, the twin sister could bring new perspectives. For example, she might have different views on the Faunus situation which could either create some tension or lead to better understanding among them.
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.
One interesting RWBY Weiss Faunus fanfiction could be one where Weiss has to team up with a Faunus character and they initially clash due to their different backgrounds. As they face challenges together, they start to understand and respect each other. For example, they might be on a mission to retrieve a powerful relic and the Faunus character's unique abilities help them get through various obstacles. Another fanfiction might explore a world where Weiss discovers she has Faunus heritage herself and has to deal with the internal struggle of accepting this new identity while still being part of the Schnee family.
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.
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.