The fanfic could start with Blake being bitten by a vampire - like creature during a solo mission. As she slowly turns, she starts to have visions of an ancient war between vampires and some powerful beings in Remnant's past. Yang, Ruby, and Weiss notice her changes and set out to find a cure. Along the way, they learn more about Remnant's history and the true nature of the vampire infection, which might be related to some sort of powerful magic or ancient technology.
When writing a RWBY vampire infection fanfic, start with a strong hook. Maybe open with a scene where a character wakes up in a strange place, already infected and not knowing how it happened. Then, research vampire lore. You can incorporate elements like a vampire's weakness to sunlight or silver into the RWBY universe in an interesting way. For instance, in Remnant, maybe vampires are weak against Dust of a certain type. Finally, have a clear plot structure. Decide whether the fanfic will be about finding a cure, or about the characters learning to live with the infection, and build the story around that goal.
One idea could be a fanfic that explores the backstories of minor characters in RWBY more deeply. For example, you could focus on a character like Velvet and create a story about her training before coming to Beacon. Another could be a crossover within the RWBY universe itself, like a team-up between different schools' teams that we haven't seen much of in the official series.
One common theme could be the struggle against the fungal infection. Characters in RWBY might be shown fighting not only their usual foes but also this new threat of the fungal infection. It could also explore themes of survival, as they try to find a way to stop the spread or cure the infected. Another theme could be the impact on the relationships between the characters. For example, if a team member gets infected, it can put a strain on the team dynamic as they have to decide whether to try and save the infected or protect themselves from the spreading infection.
Maybe it involves the characters reminiscing about their time at Beacon Academy. They could think about the classes they took, the friends they made outside their teams. Like Weiss remembering her struggles with fitting in among students from different backgrounds, and how those experiences shaped her.
One idea could be a story where Velvet discovers a new type of Grimm with unique abilities. She then has to figure out how to combat it, using her skills and the help of her friends. Maybe she could study the Grimm's behavior and weaknesses in secret to surprise everyone when she reveals her knowledge.
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.
One idea could be Ruby discovers a new form of nanotech in a hidden part of Remnant. This nanotech could enhance her Crescent Rose, giving it new abilities like shape - shifting or self - repair.
A third plot idea could be Ruby having to face an ancient vampire threat that is related to her new condition. Maybe she discovers that she was turned into a vampire as part of a bigger plan by an evil vampire lord, and she has to stop this villain while also coming to terms with her own vampirism. This could involve her learning about vampire lore and using it to her advantage.
One idea could be that Ruby, with her speed and enthusiasm, helps Pyrrha break out of her more reserved shell. For example, Ruby could drag Pyrrha into some crazy training sessions that are more about having fun than strict discipline. This would allow Pyrrha to relax a bit and not be so focused on always being perfect.
Perhaps in the fanfic, Captain America's shield could be modified with Dust technology in the RWBY world. He might also have to adjust to the different fighting styles and powers of the RWBY characters. For example, learning about Aura and Semblances and how they can be used in combination with his own abilities in combat.