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.
An ancient evil that has been awakened by the 'New Divide' could serve as the main antagonist. This evil could have been sealed away long ago and now, with the disruption in the balance of the RWBY world, it has returned. It could possess knowledge and powers that are far beyond what the main characters have ever faced, forcing them to learn and adapt quickly to have any chance of defeating it.
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.
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.
A powerful rogue ninja who has mastered a forbidden jutsu that gives him the ability to control minds could be a great main antagonist. He could use this power to turn Naruto's friends against him, creating a lot of internal conflict within the group.
The Joker could be a great antagonist. His chaotic and unpredictable nature would contrast with Naruto's more straightforward sense of 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.
It could be a new dark wizard who has emerged in Harry's new life. This wizard might be jealous of Harry's fame and want to take him down.
The Joker could also be a main antagonist in this fanfic. He could be interested in causing chaos not only in Gotham but also in the world of Wonder Woman. His unpredictable nature and love for mayhem would make him a formidable foe for the two heroes. He might come up with a plan that plays on their differences and tries to drive a wedge between them while also causing destruction.