Another possible storyline is the OC swordsman's quest for a powerful sword or a lost technique. This could involve exploring ancient ruins in the RWBY world, facing various challenges and enemies along the way, and perhaps uncovering secrets about the world's past related to swordsmanship.
Perhaps the OC is an outsider who has some information about a new threat to Vale. Glynda initially suspects the OC but as they interact more, she realizes the OC is genuine. They then form an alliance to investigate the threat, and during this process, a friendship or something more might develop between them. Their different personalities, Glynda's strictness and the OC's more free - spirited nature, could create some interesting dynamics in the story.
In some fanfictions, a knight oc could be from a different dimension where the rules of combat and magic are distinct. They come to the RWBY world and their presence disrupts the balance. Their armor might have special properties that are both an advantage and a source of mystery. They might initially be seen as a threat by Ozpin and the others, but as they interact more, they become an important part of the fight against Salem. They could also teach the RWBY characters new combat techniques based on their own knightly training.
There could also be antagonists as main characters in a sense. Maybe a powerful Grimm or a rival swordsman who has a different view on swordsmanship. The interactions between these antagonists and the OC swordsman would drive the plot forward. For instance, the rival swordsman might challenge the OC to duels to prove who is the better swordsman in the RWBY universe.
Some fanfictions might have a storyline where the characters swap personalities. Yang could become super shy and reserved, while Blake turns into a hyper - active and overly talkative character. This would lead to all sorts of comical situations as they try to adapt to their new personalities while still facing the challenges of the RWBY world. The team would have to figure out how to work together in this new, wacky dynamic.
One interesting storyline could be a character from RWBY getting injured during a battle and having to recover in the hospital. There, they might meet new allies or face unexpected challenges within the hospital setting. For example, dealing with a shortage of medical supplies due to Grimm attacks on supply routes.
One interesting storyline could be a fanfiction where the characters have to face an ancient evil that was long forgotten in the RWBY universe. This evil could be something that predates the Grimm and has powers that are completely different from what they are used to dealing with.
One storyline could be that she is on a quest to find the missing pieces of a legendary axe that once belonged to a great hero. Along the way, she teams up with the RWBY characters and they face various Grimm and villains. Another could be that she is wrongly accused of a crime in Vale and has to prove her innocence while still wielding her axe to defend herself.
In some persona 4 oc fanfiction, an OC might be a detective who is secretly investigating the strange happenings in Inaba. This OC could have their own set of skills and resources, and as they dig deeper, they discover connections between the real world and the TV world that the main characters haven't noticed yet. Their investigations could also put them in danger and lead to some intense action scenes.
Another could be an OC with the Omnitrix getting stuck in a parallel universe where the rules of the Omnitrix are different. For example, instead of being able to transform into known aliens, the OC has to figure out how to use new and strange alien forms that are native to that universe. This could lead to all sorts of adventures as the OC tries to find a way back home.
Another could be a love story within the context of the royals. An original character might fall in love with a royal heir. There could be many obstacles such as differences in social status and the expectations of the royal family. But their love overcomes all these difficulties in the end.