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 possibility for the main antagonist is a group of bounty hunters. Since Luke is now a criminal, they would be after the bounty on his head. They could be led by a particularly ruthless and skilled bounty hunter who has a personal vendetta against Luke. Maybe Luke once thwarted this bounty hunter's plans in the past, and now he sees this as a chance for revenge. Additionally, the New Republic could also act as an antagonist in a sense. They would see Luke as a threat to the peace and stability they are trying to build and would send their own forces to capture 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 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.
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.
Cinder Fall herself could be a great antagonist. Since she has a history with the Fall Maiden power, she might want to reclaim it or prevent someone else from fully harnessing it. Her manipulative and power - hungry nature would make her a formidable foe.
It could be a detective who is hell - bent on capturing Ash. This detective might have a personal vendetta or just be really dedicated to bringing criminals to justice.
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.
An organization that opposes the hero's unorthodox methods could serve as the antagonist. They might believe in a more traditional way of heroism and see the hero as a threat to their established order. They could try to stop the hero from using their unique abilities or ways of doing things.