Yang would also work well. Given her hot - tempered personality and her already existing strength, the idea of her becoming a Berserker or dealing with one could lead to some really exciting storylines. She might struggle with the increased aggression and the loss of control that comes with the Berserker state.
A new character could be the main character too. For instance, a character from a different kingdom who has a natural Berserker ability. This character could come to Vale and interact with team RWBY, causing all sorts of chaos and drama as they try to understand each other and deal with the threats that the Berserker nature might bring.
Ruby could be a great main character for a Berserker fanfiction. Her innocent and determined nature would be an interesting contrast to the savage and uncontrollable nature of a Berserker. She could be forced to embrace this side of her in order to protect her friends.
It could also be a side character like Penny. If she gets damaged in a battle and is taken to the hospital for repairs. The story could focus on how the RWBY team reacts to her situation, and Penny's own thoughts as she is being fixed. Also, it could explore the relationship between Penny and the people in the hospital who are helping her.
A grizzled veteran mercenary who has seen many battles and has a lot of combat experience. He could be cynical at first but slowly warm up to the RWBY team.
Ruby could be the main character. Since she is the protagonist in RWBY, it would be natural for her to be the one to find the ring. She could be intrigued by its power and try to figure out how to use it for good in the fight against Salem.
It could be Orochimaru. He's always been interested in dark and forbidden jutsus. He might see Naruto as an interesting experiment and use some dark magic on him to turn him into a berserker just to see what chaos he can cause in the ninja world.
A tough and experienced female bounty hunter could be the main character. She could have a no - nonsense attitude and be highly skilled in combat and tracking. Maybe she has a personal vendetta against a particular group in the RWBY universe, like a band of thieves who stole from her family.
Perhaps Professor Ozpin could be turned into a mad scientist in a different take on the story. His vast knowledge and access to resources could be used in a more sinister way. He might start to believe that the only way to save Remnant is through extreme and unethical experiments. Another option could be a young prodigy who is corrupted by the power of knowledge and becomes a mad scientist. This character could be initially a friend of Team RWBY but then turn on them as his experiments spiral out of control.
The main character would most likely be the new thief. They could be an original character created just for this fanfiction.
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.
Ruby Rose could be the main character. Her optimistic and fearless nature would make her a great fit for a story involving the Green Lantern. She's always eager to take on new challenges, and if she were to get involved with the Green Lantern's power, she would be quick to try and master it to protect her friends and the world.
It could be an original character. For example, a young girl who discovers she is half - Grimm after an accident. She has to hide her true nature from others while trying to understand her powers.