Blake could also work well as the main character. Her past experiences with the White Fang and her ability to think independently would make her exploration of the Silver Flame and its implications very interesting. She might be more cautious and investigative about it, given her nature.
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.
A new character could be introduced as the main character. This character could be an outsider who stumbles upon the Silver Flame. They would have no prior knowledge of the RWBY world's politics or powers, and their journey of discovery, while dealing with the existing RWBY characters, could make for an exciting fanfic.
The Silver Flame is surely a main character. There could also be characters related to the Silver Flame, like their comrades or mentors who might have influenced their growth and abilities, which in turn helped them to defeat the top huntsman. And of course, the top huntsman is a key character as the one being defeated.
Perhaps he is a spirit from an ancient time. The silver flame is a remnant of an old magic that has resurfaced. He has been awakened in the present day of RWBY's world and is trying to understand this new world while also dealing with the power of the silver flame that he doesn't fully comprehend.
In the fanfic, the Silver Flame could be representing an underdog force. Their victory over the top huntsman might symbolize the triumph of the underestimated. They could have used their wits and the element of surprise. For example, they might have feigned weakness at first to make the top huntsman overconfident, and then launched a sudden and powerful counter - attack using their unique skills and abilities.
It could be a powerful Grimm that has a special connection to the character in the trailer. Maybe this Grimm has been hunting them for a long time and is finally about to catch up.
Another plot idea could be a character who is mysteriously connected to the Silver Flame. They could have powers that are enhanced by it and have to hide this fact from others for some reason, like fear of being experimented on. For example, a new student at Beacon Academy discovers this connection and has to navigate through the challenges while keeping it a secret.
One common theme might be power and transformation. The silver flame across the body could symbolize a new power or a transformation that the character is undergoing. Another theme could be struggle, like the character trying to control or come to terms with this new silver - flamed aspect of themselves.
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.
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.