A group within the Hunter Association that has gone bad could serve as the main antagonist. They might be secretly conducting unethical experiments using Nen and Pokkle and Ponzu stumble upon their operations. To keep their secrets, this group would target Pokkle and Ponzu, trying to frame them for crimes within the association so that no one would believe their accusations against the group.
It could be a rogue hunter who has his own twisted ideas about the use of Nen. This hunter might see Pokkle and Ponzu as obstacles in his quest for power and knowledge. He could try to capture them to steal their Nen abilities or the information they have about certain areas of the Hunter x Hunter world.
Maybe a powerful and ancient Nen - using creature that Pokkle and Ponzu accidentally awaken. This creature has been dormant for centuries and blames the hunters for its slumber being disturbed. It would use its formidable Nen - based powers to try and destroy Pokkle, Ponzu, and any other hunters who get in its way. It could have the ability to manipulate the environment and the minds of those around it, making it a very difficult adversary to face.
One idea could be that Pokkle and Ponzu discover a hidden area in the Hunter x Hunter world filled with unique and powerful creatures. They have to team up with some unlikely allies to survive and document these new finds.
There could also be a corrupt official within the Hunter's Guild. This person doesn't like the way the Handler and Hunter are operating, perhaps because they are too independent or they are getting in the way of some underhanded scheme the official has. So, this official uses their power to try and undermine the duo, from withholding resources to sending them on extremely dangerous and near - impossible missions.
Fanfiction.net is also a good place. There are many dedicated Hunter x Hunter fans who post their works there. Look for the tags related to Ponzu. Sometimes, you might need to dig a little deeper as there are a lot of stories, but it's definitely worth it.
One great Ponzu Hunter x Hunter fanfiction could be 'Ponzu's Unseen Adventure'. It might explore her backstory more deeply, like her reasons for entering the Hunter Exam in the first place. Another could be 'Ponzu and the Mysterious Beasts', where she encounters strange creatures and uses her unique skills to survive.
A 'ponzu oc hunter x hunter fanfic' is likely a fan - created story. Ponzu could be an OC that the author has made up. In the Hunter x Hunter world, this fanfic might involve Ponzu getting involved in the Hunter Exam, facing off against the likes of Hisoka or other examiners. They could be on a quest to find rare items or secrets in the world, which is full of strange creatures and hidden powers.
One way is to expand on her skills. Maybe she has a talent for something other than what we've seen in the original series, like being an excellent tracker or having a deep understanding of the history of certain artifacts in the Hunter x Hunter world. This new skill could open up new plotlines and interactions for her.
Perhaps an ancient evil entity that was once sealed away by Deer Dipper's ancestors. It has now been awakened and sees Deer Dipper and Hunter Bill as threats. It uses dark magic and mind - control to try to turn them against each other so that it can regain its full strength and take over their world.
There could be a group of poachers who are not just after regular game but are also involved in some dark magic. They oppose the hunters and especially Percy when he tries to stop their illegal and dangerous activities. They might have powerful backers or magic items that make them a formidable foe for Percy and the other hunters.
It could be a powerful devil who has the ability to control minds. This devil could manipulate some of Naruto's friends and turn them against him, creating a lot of internal conflict within Konoha.
Sorry, I'm not really familiar with specific 'ponzu oc hunter x hunter fanfic' works. But you can try searching on fanfic platforms like Archive of Our Own or Fanfiction.net. There you can use the relevant tags to find similar works.