There could be an organization in the fanfiction that opposes the lone huntsman. They might see him as a threat to their plans or ideology. This organization could have a hierarchy of powerful members, each with their own set of skills and abilities, making them a formidable foe for the lone huntsman.
It could be a powerful and mysterious figure from the shadows. Maybe a rogue huntsman gone bad who has his own agenda against the lone huntsman. This antagonist might have a vendetta for some past event.
A new dark wizard or witch could be created specifically for this fanfiction. This character could be interested in using the Huntsman's skills for their own evil purposes. If the Huntsman refuses, they become enemies. This antagonist could have powers related to dark forest magic, something that the Huntsman is familiar with but in a more malevolent form.
Well, the relationships in this fanfiction can be quite engaging. Since he is a lone huntsman, his interactions with other characters from RWBY are likely to be different. He might start as an outsider but gradually form unique bonds. Maybe he has a reluctant alliance with a team at first, and over time, they come to respect and rely on each other in unexpected ways.
The idea of a lone huntsman in the RWBY universe is appealing because it allows for a different perspective. Instead of following the main RWBY teams all the time, we get to see how an individual oc can interact with the world and its challenges.
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.
Sure. One popular 'lone huntsman oc rwby fanfiction' is 'The Lone Huntsman's Journey'. It follows the adventures of an original character as a huntsman in the RWBY universe. The story has great character development and exciting battles.
Perhaps an ancient and malevolent sorcerer who has been awakened from a long slumber. The sorcerer wants to use Percy's powers for his own dark purposes. He sets traps and sends magical creatures to stop Percy as he tries to figure out what the sorcerer is up to and how to defeat him. The sorcerer would be a very tricky antagonist as he can use all kinds of magic that Percy has never faced before.
It could be a rogue ninja who has developed a new and extremely deadly jutsu. This ninja has a vendetta against Konoha and was able to take out most of the ninja there, leaving Naruto as the lone survivor.
I'm not sure as I haven't read it, but likely there will be a Huntsman as one of the main characters. There might also be other RWBY characters who are either helping or hindering the Huntsman's journey in the Void.
There could be an ancient evil entity that was sealed away long ago in Vacuo. Due to some recent events, it starts to break free. It corrupts the minds of some of the King's men and tries to use them to get rid of the King and RWBY, who are the only ones who can stop it from fully emerging and causing destruction.
It could be a power - hungry general from one of the warring factions. Someone who is willing to sacrifice anything and anyone to achieve victory.