In a 'team RWBY as maiden fanfiction', perhaps there is a situation where the Maidens' powers are being drained by an unknown force. Team RWBY, now as Maidens, has to go on a quest to find the source of this problem. Along the way, they face many challenges. Ruby uses her Maiden - like speed to scout ahead, Weiss her ice powers to cross difficult terrains, Blake her shadow - like Maiden powers to hide from enemies, and Yang her fiery Maiden powers to fight off foes. They also have to figure out how to combine their powers effectively to overcome the final obstacle.
One idea could be that Ruby discovers she has the power of a Maiden in a moment of great danger. She then has to learn to control it quickly while still leading her team. Another could be that Weiss, as a Maiden, uses her ice powers in new and unexpected ways, like creating entire ice fortresses to protect her team from a powerful enemy. Maybe Blake, as a Maiden, is able to use her powers to communicate with the Grimm in some strange way, leading to a new understanding of them.
One idea could be exploring the backstory of the Fall Maiden before she passed on the power. Maybe she had a secret love interest that was a threat to the order of Maidens. Another could be a story where the new Fall Maiden struggles to control her powers and has to learn from an unexpected mentor, like a character from the criminal underworld who has unique knowledge about the nature of magic.
A great plot could be centered around the Summer Maiden's relationships. For example, she might have a forbidden love with a character from the other side, like a member of Salem's faction. This would create a lot of internal conflict as she has to balance her feelings with her duty as a Maiden. Or perhaps she has a deep friendship with a character who is in danger, and she has to use her powers to save them from a great threat.
A plot could revolve around a new, powerful enemy that specifically targets Team JNPR. This enemy has studied their weaknesses and uses that knowledge to try and take them down one by one. The team then has to overcome their individual insecurities and grow stronger as a unit to defeat this foe. For example, Nora might have to face her fear of being alone, while Ren has to deal with his struggle to open up more emotionally.
One idea could be that the OC team is on a mission to retrieve a powerful relic lost in a dangerous, abandoned Grimm-infested city. Each member of the team has a unique skill that they have to use to overcome different obstacles, like one being able to manipulate ice to freeze the Grimm, another with super speed to outrun them, etc.
A possible plot could involve Pyrrha having to face off against other powerful characters who want to steal her Maiden powers. She would have to use her combat skills, which she's already known for, in combination with her new Maiden abilities. Maybe she teams up with some unexpected allies, like a group of outcast Huntsmen who have their own reasons for wanting to protect the Maiden powers.
One idea could be a shrine maiden who has the power to communicate with the spirits of past shinobi. She teams up with Naruto to solve a mystery that threatens the Hidden Leaf Village. The spirits might provide clues that only she can understand.
I think Ruby could be the most likely. She has always been at the forefront of the team, with her leadership skills and strong will. Her silver eyes already give her an edge, and as a Maiden, she could use her speed and new powers to protect her friends even better. Also, her innocent - looking exterior but strong - hearted interior would make for an interesting contrast with the powerful Maiden powers.
One idea could be Ruby discovers she is the Winter Maiden in a moment of great danger. As she tries to control her new powers, she has to face off against Salem's minions who are after the power of the Maiden. Ruby's friends, Weiss, Blake and Yang, are initially shocked but quickly support her in her new role.
One idea could be Kirby arriving in Remnant and being mistaken for a new type of Grimm at first. Then, as the characters get to know Kirby, they realize his unique abilities can be a great asset in battles against Salem's forces.
One idea could be that the RWBY team decides to start a new workout routine to improve their combat skills. They face challenges like over - training and have to find a balance. Another could be a character - specific story, for example, Yang working out to overcome an injury and regain her strength. Or perhaps the team trains with a new instructor who has very unorthodox methods.