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.
Well, in a 'Power of a Single Rose RWBY Crossover Fanfic', perhaps the single rose is a symbol of a long - lost magic. The fanfic might revolve around a character finding this rose and unlocking powers that were forgotten in the RWBY world. The crossover part could bring in new races or technologies from another series that are related to the power of the rose. For example, if it's a crossover with a fantasy series, there could be elves or dwarves interested in the rose's power.
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.
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.
Most likely Ruby. Since 'rose' is in the title and Ruby is often associated with roses in RWBY. The 'thorn' part could be something or someone that challenges her, like a new enemy or an internal struggle she has to face.
Typically, the main characters would still include Ruby, Weiss, Blake, and Yang from the RWBY series. However, in the Enchanted Rose fanfic, there might be new characters introduced who are somehow related to the enchanted rose, like a guardian of the rose or a character who is cursed because of the rose.
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.