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.
Well, one key theme could be power struggles. In the RWBY and Warhammer universes, power is a major factor. In an enslaved scenario in a fanfic, it might show how characters deal with having their power taken away or fight against their enslavers. Another theme could be survival. Characters would have to find ways to stay alive in a harsh, enslaved situation. And there could also be a theme of resistance, as the RWBY characters are known for their strong wills and would likely resist the enslavement.
Well, this kind of fanfiction might be exploring themes that are a bit controversial. It could be that the writer wants to show a different side of the RWBY characters by putting them in a difficult situation like enslavement in the world of Total Warhammer. However, it also depends on how the author handles it. If not done carefully, it could be misinterpreted. For example, if it's too dark and doesn't have any redeeming qualities in the story, it might not be well - received by all fans.
The main characters would surely be Cinder, who is in a position of power in this fanfic. And then there are Ruby Rose, the fearless leader of Team RWBY, Weiss Schnee, the heiress with her own set of skills, Blake Belladonna, the former White Fang member, and Yang Xiao Long, the strong fighter. They are the ones who will be at the center of the story as they face the situation of being enslaved by Cinder.
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.
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.
You can try looking on some well - known fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fan - created works, and there's a good chance you might find this particular fanfiction there.
First, know the source materials well. You need to understand the characters, settings, and rules in both RWBY and Warhammer. This will help you create a believable story. Second, focus on the emotions of the enslaved characters. Show their pain, their hope for freedom, and their anger towards their oppressors.