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.
There's also a possibility that it could be a new character created specifically for this fanfic. The author might have wanted to introduce someone who has a unique ability or weapon that allows them to make the 'winning shot' and be the center of the story.
Since it's a RWBY fanfic titled 'the winning shot', it could be about a variety of things. It could be about a particular fight scene where one of the characters makes a shot that turns the tide of the battle. For example, Blake might use her weapon to make a shot that disables a powerful Grimm. Or it could be about a competition within the RWBY story, like a tournament, and a character makes a winning shot, whether it's a physical attack or a strategic move that outwits their opponent.
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.
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.
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.
Ruby could be a great main character in such a fanfic. Given her leadership skills and her silver - eyed warrior powers, a time stop ability could be an interesting addition to her arsenal. It would add a new layer to her character development as she learns to control and use this power for the right reasons.
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.
Blake could be an excellent main character for a stop - time fanfic. Her past and her struggle with identity could be intertwined with the power. For example, she could use the time - stopping ability to face her inner demons and come to terms with her past actions.