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.
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.
Well, in some fanfictions, the Shinigami might save Naruto by using its special powers to reverse a fatal curse or injury. For example, it could extract a harmful entity that was about to kill Naruto from his body.
Maybe the Shinigami uses its special powers to heal Naruto's severe injuries. For example, it could transfer some of its own spiritual energy to Naruto to revive his fading life force.
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's always been after Naruto for his special powers. He might set a trap for Naruto, but Gaara intervenes and saves him. Orochimaru's motives of immortality and his dark jutsu make him a great antagonist for this kind of story.
One consequence could be that Naruto has some new powers or abilities. Since the Shinigami is a very powerful and mysterious being, its intervention might leave a mark on Naruto in the form of newfound powers.
Obviously, Naruto would be one of the main characters since it's in the title. And the Shinigami who gives the gift is likely to be another important character.
Naruto is surely the central character in this fanfiction. Given the title, the Shinigami is another key figure. It's possible that characters like Kakashi could also play important roles. Maybe he serves as a mentor to Naruto in understanding the powers and responsibilities that come with being the champion of the shinigami. Also, villains or antagonists related to the shinigami's realm might be introduced as main characters to oppose Naruto.