In a fanfiction where Roman joins Beacon, he might actually have a change of heart. Perhaps he sees the good in the students and the purpose of the Huntsmen and Huntresses. He could start to use his skills, which were previously used for criminal activities, to help the school. For example, he could use his knowledge of the criminal underworld to protect Beacon from outside threats.
Well, one possibility could be that Roman joins Beacon under false pretenses. Maybe he's trying to steal some valuable Dust or intel from the school. He could pretend to be a reformed criminal, fooling some of the students and teachers at first. But as the story progresses, his true motives start to surface and the characters have to deal with the betrayal.
If Ruby joins Roman in the fanfiction, it might start with Ruby being tricked by Roman. He could pretend to have some information that Ruby desperately needs, like the location of a powerful relic that is about to be used for evil. Once she's with him, she realizes that his ways are completely different from hers. However, she decides to stay for a while to gather more intel. During this time, she has to constantly resist his attempts to make her more like him in terms of being less rule - bound.
Neo could bring a new dynamic to Beacon. She might be initially shunned because of her association with the criminal underworld. But as time goes on, her unique skills in combat and deception could prove useful in battles against Grimm, and she could earn the respect of her peers.
She could be a wildcard. Since Neo is quite unpredictable, her role might be to disrupt the normal order at Beacon. For example, she could start small rivalries between different teams just for fun.
One possible plot could be that the characters are able to prevent the fall through better communication and teamwork. They might discover the enemy's plans earlier and work together to strengthen the defenses of Beacon. For example, Ruby could use her speed to quickly spread warnings, and Weiss could use her glyphs to fortify key areas.
If Ruby joins Salem in the fanfiction, it could be a very dark turn for Ruby. Salem might manipulate Ruby into believing that her methods are the only way to bring about a new order. However, Ruby's friends would never give up on her. They would try to rescue her, and in the process, Ruby might have a change of heart and turn against Salem, leading to an epic showdown.
Ruby would probably be very suspicious at first. She knows Roman from his criminal activities and would find it hard to trust him right away. But being the kind - hearted person she is, she might give him a chance to prove himself.
One possible reason could be that they have their own personal goals that conflict with what Beacon offers. For example, they might want to pursue a different type of combat training or they have a family matter to attend to first. Maybe they've heard rumors about Beacon that make them hesitant, like strict rules that would limit their freedom.
One possible plot could be that Naruto, disillusioned with Konoha for some reason, perhaps feeling misunderstood or not fully accepted. He is then approached by Orochimaru or one of his followers from Otogakure. They offer him a place where he can explore his power without the constraints he feels in Konoha. Naruto, being a character who always craves power to protect his friends, decides to give it a shot. Once in Otogakure, he starts to learn different jutsu and fighting styles. However, he soon realizes that Otogakure has its own dark secrets and he has to decide whether to stay and try to change it from within or return to Konoha.
If Ruby joins Roman, her character could develop in a more rebellious way. She's always been a bit of a rule - follower in the context of the Huntress code. But being with Roman would expose her to a different set of rules, or rather the lack of them. This could lead her to break some of the traditional Huntress norms. However, she would also be constantly fighting against Roman's more amoral tendencies, which would in turn strengthen her own moral compass in some ways. For example, she might have to stand up to Roman when he wants to do something really bad, like harm an innocent bystander for his own gain.
He could bring new skills and knowledge. For example, his apparating ability could be useful in combat or for quick escapes in RWBY's dangerous situations. Also, his knowledge of magic could potentially be combined with the Dust - based technology in RWBY. He might start teaching some students about different forms of magic, which could change the way battles are fought at Beacon.