The main antagonist could be a corrupted version of the Shinigami itself. The Shinigami that Naruto makes a deal with might be one of the good ones, but there could be an evil counterpart that doesn't want Naruto to succeed. This corrupted Shinigami would use all its powers and minions to stop Naruto from achieving what he set out to do with the deal.
One possible plot could be that Naruto makes a deal with the Shinigami to gain more power to defeat a powerful enemy. In exchange, he might offer his own lifespan or something precious to him. Maybe he needs this power to save his village from a new, overwhelming threat like an ancient evil that has awakened.
Maybe it's a new villain created specifically for the fanfiction. This villain could have powers that are extremely dangerous to Naruto, like the ability to drain his chakra completely. So the Shinigami has to step in to save Naruto from this new and powerful foe.
A powerful rogue Shinigami could be the main antagonist. This Shinigami might be jealous of the Shinigami King's power and wants to overthrow him. He sees Naruto as a threat because of his connection to the king and tries to stop Naruto from fulfilling any tasks or missions related to the king. He could also be causing chaos in the living world by releasing dangerous spirits or curses.
It could be an evil Gundam pilot who is jealous of Naruto's unique abilities because of the Shinigami mark. This pilot might see Naruto as a threat to his own power and status in the Gundam world and constantly tries to undermine him in battles.
Another possibility is that there could be a rogue shinigami who doesn't like the fact that one of their kind is raising a human. This rogue shinigami could try to undermine Harry's growth and even try to harm him or the shinigami who is raising him.
It could be Orochimaru. He might have heard about the keyblade and wants it for his own evil purposes. He could try to steal it from Naruto, believing that with the keyblade's power, he could become invincible. Orochimaru is always looking for new sources of power, and the keyblade would be a very tempting addition to his arsenal.
It could be Naruto himself. Imagine Naruto encountering a rogue Shinigami and having to deal with the chaos it brings. He would be the perfect main character as he is always eager to face challenges and protect his friends and the village.
Some might think Naruto could be a Shinigami fanfiction because of the presence of certain death - related elements. For example, the concept of sacrificing one's life for a greater cause, which is somewhat similar to the Shinigami's association with death. Also, the idea of powerful beings that can manipulate the balance of life and death in the Naruto world, like the Sage of Six Paths, might give the impression of Shinigami - like entities. But these are just loose associations and not evidence of it being a fanfiction.
The devil could be a new, mysterious character created specifically for the fanfiction. It might be an ancient evil entity that has been lurking in the shadows of the Naruto world for ages.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.