A group of hunters could be the main antagonists. They might be after Pitch Black because they sense that he's not an ordinary cat. They could have some connection to the dark forces that Pitch Black was originally associated with. They want to capture him and use his powers for their own gain, not caring about what will happen to Pitch Black in the process.
Well, the plot could involve Pitch Black, now in cat form, trying to navigate a world full of strange new sensations. Maybe he's on a quest to regain his original form. He could be befriending other animals who help him in his journey, and along the way, he discovers the simple joys of being a cat, like chasing mice or lazing in the sun.
Well, perhaps Pitch Black, now as a cat, could be on a quest to find a way back to his original form. He might encounter other magical creatures in the cat world who either help or hinder him.
There could be a group of mean - spirited fairies. They see Sakura as an intruder in their territory, which might be a beautiful garden that Sakura often visits. To drive her away, they use their fairy magic to turn her into a cat, thinking that she will be less of a threat in that form. And they keep thwarting her attempts to turn back into a human throughout the fanfic.
Another option is that Pitch Black himself could be the main antagonist in a different way. He could start off as an anti - hero but then be corrupted by an even greater power of darkness. This new - found power makes him go overboard with his fear - spreading, and Jack Frost has to find a way to bring back the old Pitch Black or at least stop his rampage. This would create an interesting internal struggle within Pitch Black as well as an external conflict between him and Jack.
It could be Orochimaru. He might see Naruto's new form as an interesting experiment or a threat to his plans. He could try to capture Naruto to study her newfound female - related powers or use her in some dark jutsu.
The main antagonist could be the one who turned Percy into a monster in the first place. Maybe it's a vengeful god who feels Percy has wronged them in some way and decides to punish him by turning him into a fearsome beast.
It could be a powerful vampire hunter who believes Naruto, as a new vampire, is a threat to the balance between humans and vampires. This hunter might be from a long line of slayers and is determined to eliminate Naruto before he fully embraces his vampire side.
Well, in his cat form, Pitch Black could become more territorial. He might hiss at other animals or people who come too close to his 'domain'. His once menacing presence could be translated into a more stealthy and watchful nature. He would still be cunning, but now he would use his cat - like abilities to his advantage, like sneaking up on things. And perhaps his patience would increase, as cats are known to wait for the perfect moment to pounce.
There might be a jealous friend or acquaintance who feels that Stiles, as a girl, is getting too much attention. This person could then start spreading rumors or causing trouble for Stiles in an attempt to bring her down.
It could be a dark wizard who has a long - standing vendetta against Ron's family. This wizard might see turning Ron undead as a way to get revenge on the Weasleys. He could be lurking in the shadows, trying to make Ron's undead state even worse by casting more dark spells on him.