I think Weiss could be an interesting main character with an OP power. What if she had the power to create ice that could never melt? She could build entire fortresses out of it during battles, or freeze Grimm in their tracks permanently. It would be a huge upgrade to her existing ice - based abilities and give her a more dominant role in the story.
Yang could also be a good choice. If she had an OP power like super strength that was even more extreme than her current semblance. For example, she could lift mountains or punch through the toughest Grimm defenses with ease. This would add a new level of badassery to her character and change the way she approaches battles.
Ruby could be a great main character with an OP power. Imagine if she had the power to teleport short distances instantly. It would enhance her already amazing combat skills and make her even more effective in battle. She could use this power to surprise her enemies and quickly get to her teammates in need.
Another possibility is a new character introduced specifically for this fanfic. This character could be an outsider to the RWBY world, coming from the world of the crossover. They find the single rose and have to navigate the RWBY universe while dealing with the power the rose gives them. It could be a really interesting way to explore both universes from an outsider's perspective.
One idea could be a character from our world being reincarnated into the RWBY universe with an OP power like the ability to control time. They could use this power to prevent major disasters in the story, like stopping Salem from causing chaos in the early days. Another could be a character who has the power to instantly heal any wound, which would change the dynamic of battles as they could keep their teammates alive no matter how severe the injury.
Perhaps it could also focus on some of the villains in RWBY but from a different perspective. They might be the ones who are also trying to harness or understand a certain power within themselves, which could add an interesting twist to the story as they are usually seen as just the antagonists.
Cinder Fall might be a likely candidate. She has a connection to fire already, so it would make sense for her to be involved in brewing hellfire. It could be part of her plan to gain more power and cause more chaos in the RWBY world.
Yang could be the main character too. She's bold and strong. If she gets caught in a snake - related situation, like being trapped in a lair of snake - like creatures while on a solo mission, it would show her determination to fight her way out and protect those she cares about.
Blake could also work well as the main character. Given her past and her connection to the White Fang, she has a lot of internal conflict. The God of Darkness could either prey on that conflict or offer her a way to resolve it in a way that is both tempting and dangerous. This would make for an interesting exploration of her character.
Ruby could be a great main character for a 'rwby silver flame fanfic'. Given her natural leadership and combat skills, she could be on a quest to understand and control the Silver Flame, perhaps believing it could be a new weapon against Salem.
Oscar could be an interesting main character for a time travel RWBY fanfic. As Ozpin's current host, he has a connection to a long history of events in Remnant. Time traveling could allow him to learn more about Ozpin's past directly, and he could also use his new - found knowledge to change things for the better in the present and future of Remnant.
It could be Ruby. She's often in the thick of the action and her skills with Crescent Rose could be the key to making a 'winning shot' in the story.
It could be Sam Fisher from Splinter Cell. He's a well - known and highly skilled agent. His experience in stealth operations would make him a great fit in the RWBY universe, where he could face new challenges like dealing with the Grimm while also using his skills to help the RWBY characters.