It could be Sasuke. If Naruto becomes a wraith, Sasuke might see him as a threat to his own power - seeking goals. Sasuke could try to eliminate Naruto in this new form, thinking that Naruto is now too dangerous or has strayed from the path he believes in.
Maybe it's Kabuto. He's always been interested in experimenting with different powers. With Naruto as a wraith, Kabuto might want to capture him and study his new form to gain an edge in the shinobi world. He could set traps and use his medical and ninja skills to try and subdue Naruto, creating a lot of conflict in the fanfiction.
It could be Orochimaru. He's always been interested in dark and forbidden jutsus. He might see Naruto as an interesting experiment and use some dark magic on him to turn him into a berserker just to see what chaos he can cause in the ninja world.
It could be Orochimaru. Since he has always been obsessed with immortality and power, he might see Naruto's new - found immortality as something he wants to steal or study. He could use his dark jutsus and his army of followers to try and capture Naruto.
It could be a new, powerful ninja from a far - off land who wants to destroy Konoha. This ninja might have a grudge against Naruto specifically and forces him to become the shield to protect the village.
Perhaps it's a jealous ninja from another village. This ninja has always been envious of Naruto's power. So when he gets the chance, he uses a dark magic to turn Naruto into a wolf. His goal is to make Naruto weak and lose his status among the ninja community.
In this fanfiction, Naruto might become a wraith as a result of some dark jutsu gone wrong. As a wraith, he could have enhanced speed and stealth. He may struggle with his new form at first, as it goes against his usual open and direct fighting style. For example, he might accidentally pass through his friends when trying to give them a high - five, leading to some comical moments while also exploring the more serious implications of his transformation.
Perhaps it's a rogue Shinigami who doesn't want Naruto to figure out how to control his new form. This Shinigami might fear that Naruto could disrupt the balance between Hollows and Shinigami if he becomes too powerful in his hollow state. So, he tries to stop Naruto at every turn, creating a lot of conflict in the fanfiction.
It could be Orochimaru. He has always been interested in experimenting with new forms and powers. If Naruto becomes a monster, Orochimaru might see him as a perfect specimen to study or use for his own evil plans.
It could be a powerful Hollow that has somehow broken free from the normal Hollow hierarchy and is causing chaos in the Soul Society. This Hollow might have a special ability that makes it difficult for the Soul Reapers to defeat it, and Naruto, as the new Soul Reaper, has to figure out a way to stop it.
It could be someone who doesn't like the idea of Aayla serving Naruto. Maybe a rival of Naruto who sees Aayla as a way to get to Naruto or disrupt his life.
Draco Malfoy could be a main antagonist. Since Harry becoming a prefect is a sign of prestige and power, Malfoy, who is always jealous of Harry, would try to undermine him. He might spread false rumors about Harry misusing his prefect powers or try to get him in trouble with the professors.