It could be Adam Taurus. He's a well - known and very radical member of the White Fang in the RWBY universe. His extreme views and violent methods would put him directly at odds with Ruby's more peaceful and inclusive approach.
The main antagonist could also be a group of hard - line White Fang members who oppose any sort of change or cooperation with the outside world. They see Ruby as an intruder and a threat to their way of life and are determined to stop her at all costs, even if it means going against the more moderate members of their own organization.
It could be Salem. She has always been interested in the power of the silver eyes, and Ruby having those eyes makes her a prime target. Salem might send her minions to kidnap Ruby, which would lead to her disappearance.
It could be a powerful Grimm - like being that has evolved beyond the normal Grimm. This antagonist might have the ability to think and strategize like a human, making it a much more dangerous foe for Ruby and her team. It could be leading the Grimm attacks in a more organized way, targeting specific areas to cause maximum chaos.
There's a possibility that a new character could emerge as the main antagonist. Ruby's time - traveling could disrupt the normal flow of events, and this might create a new power - hungry or revenge - seeking character. This new antagonist could be someone who blames Ruby for the changes in the timeline and wants to stop her at all costs.
It might be Salem. Since Ruby's silver eyes are a major threat to her plans, an early activation could draw Salem's immediate attention. She would then send her most powerful minions to deal with Ruby.
If Ruby turns evil, she could become her own antagonist in a way. Her former self, the Ruby that was good, could be seen as what she is now fighting against. Her friends would also become antagonists as they try to stop her from causing more harm.
Perhaps it could be an evil version of one of Ruby's own relatives. Like an alternate version of her father or mother who has been corrupted by some dark force. They would then raise Ruby with their evil values, making her believe that what they do is right and leading her down a dark path.
The main antagonist could also be a new character created specifically for the fanfic. This character could be an ancient evil that has been waiting for an opportunity to corrupt a powerful Huntress like Ruby. They would then use Ruby's newfound evil nature to further their own dark plans, and Weiss, Blake, and Yang would have to face both Ruby and this new threat.
One idea could be Ruby trying to infiltrate the White Fang to understand their motives better. Maybe she disguises herself as a new recruit and has to navigate their complex hierarchy and various factions within the group.
A new character created specifically for the fanfiction could be the main antagonist. For example, a powerful being from another dimension who is attracted to the chaos in Remnant caused by the combination of RWBY and Starkiller's presence. This character could have abilities that are a threat to both Starkiller and the RWBY characters, forcing them to work together in ways they haven't before.
The White Fang could be antagonists, seeing Ruby as a symbol of human - made abominations (in their view) with her cyborg body. They might target her to make a statement against humans.