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.
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.
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.
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.
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.
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.
Obviously, the RWBY team - Ruby, Weiss, Blake, and Yang - are often involved. Ruby might be the most likely to initially trust Roman if he shows signs of changing. Yang could be more skeptical, leading to some interesting conflicts.
If Neo were to join Team RWBY in fanfiction, it would be a huge change. Team RWBY is based on trust and friendship, and Neo has a history of being on the wrong side. But her unique fighting style could bring new strategies to the team. Maybe she could teach the others how to better deceive their opponents, like she does with her illusions. Also, her presence could create some internal conflicts as the team tries to figure out if they can really trust her.
Power dynamics is also a common theme. Roman is often seen as the more dominant figure, but Neo has her own strengths. Fanfictions play around with this power dynamic, sometimes showing Neo as the one who actually has more control in certain situations. Also, the theme of mystery is there as their pasts are not fully explored in the original series, so fanfictions often try to fill in the gaps and create a more complete picture of who they are.