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.
Another option for the main character could be a new character created specifically for this fanfiction. This character could be a hybrid of the two universes, having some connection to the RWBY world already but also being a potential candidate for a Green Lantern. This would allow for a fresh perspective and new storylines that don't rely too much on the existing RWBY characters' established arcs.
Blake Belladonna could also be a main character. Her complex past and her fight for justice in a different way from the others could be explored in relation to the Green Lantern. She might be initially hesitant about the power of the ring but then realize how it could help in her own personal battles as well as the greater good in the RWBY world.
Hal Jordan could be the main character. He could be in a mentoring role for the Young Justice members, sharing his experiences as a Green Lantern. His cocky and confident personality would add an interesting dynamic to the story as he tries to teach the younger heroes.
Maybe an alien from the Green Lantern universe who has a vendetta against the Green Lantern Corps decides to target Naruto's world. This alien believes that by causing chaos in the Naruto universe, he can draw out the Green Lanterns and destroy them. He has powerful weapons and abilities that are a challenge for Naruto and his friends to overcome.
Maybe an alien race that the Green Lanterns have underestimated. They see Naruto as an easy target since he's being neglected. But Naruto turns out to be more of a challenge than they thought. This alien race could have some sort of power that nullifies the Green Lantern rings, but Naruto's ninja skills are still effective against them.
Hal Jordan's sidekick could be a young pilot in training. Since Hal is also a pilot, this sidekick would share that passion. They could be someone who idolizes Hal and is constantly trying to prove themselves. In a fanfiction, they could face challenges related to living up to Hal's reputation while also carving out their own identity as a hero.
The world - building aspect is quite interesting. Fanfiction often has to find a way to blend the two universes seamlessly. This means creating new rules for how the Green Lantern powers work in Remnant, or how the RWBY characters might be recruited into the Green Lantern Corps. It's exciting to see how the writers expand on the existing RWBY lore and incorporate Green Lantern elements. For instance, they might explain how the Dust in RWBY could be related to the power source of the Green Lantern rings, or how the Grimm could pose a new threat to the Green Lanterns.
Perhaps Atrocitus. He might view Xander as a symbol of the Green Lantern Corps that he despises. He could come to Xander's world to cause destruction and try to eliminate Xander, believing that by doing so he is weakening the Green Lanterns. Xander would then have to use his wits and the power of the Green Lantern ring to stop Atrocitus' rampage.
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.
Yes, Green Lantern is definitely a DC Comics character. He's one of the well-known heroes in the DC universe.