It could be a powerful Grimm that has a special connection to the character in the trailer. Maybe this Grimm has been hunting them for a long time and is finally about to catch up.
Another option for the main antagonist could be a rival hunter or fighter. Someone who has a different ideology than the main character from the Silver Trailer. They might see the main character as a threat to their own goals and try to stop them at every turn.
Dr. Eggman could be a great antagonist. He might be creating a new weapon or device that could disrupt the balance between time (which is relevant to Silver) and space (relevant to Shadow). His actions force Silver and Shadow to work together to stop him.
The main antagonist could be a corrupted figure from the RWBY universe who has allied with some of the darker forces in the Warhammer world. Maybe a character who has been tempted by power and is now using their knowledge of RWBY to help enslave the others. Or it could be a new, made - up character in the fanfic who has a personal vendetta against the RWBY team and uses Warhammer - style enslavement methods.
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.
It could be a powerful Grimm that has evolved during the 'New Divide'. This Grimm could have the ability to manipulate the minds of other Grimm, making them even more dangerous. It could target the main characters specifically, seeing them as a threat to its newfound power.
There could be an ancient Grimm entity that has been dormant in the Pacific for ages. Due to some recent events, it wakes up and starts wreaking havoc. It has the ability to corrupt other Grimm and make them more powerful and intelligent. This entity would be a formidable antagonist for the RWBY team as they try to stop its reign of terror in the Pacific region.
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.
Ruby could be a great main character for a 'rwby silver flame fanfic'. Given her natural leadership and combat skills, she could be on a quest to understand and control the Silver Flame, perhaps believing it could be a new weapon against Salem.
Roman Torchwick would be an obvious choice. He already has a criminal background in the RWBY series. He could return with a new and more devious plan, perhaps teaming up with other villains to cause chaos.
One idea could be exploring the backstory of the character in the Silver Trailer. Maybe show how they got their unique weapon and the training they went through. Another could be creating a prequel that leads up to the events in the trailer, like their first encounter with the Grimm.
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.