Maybe it's a rogue spirit that has escaped from the realm of the God of Death. This spirit is angry at the God of Death and Naruto for some reason and wants to destroy everything in the Naruto universe. It has powers that are similar to the God of Death's but is using them in a very destructive way.
It could be an evil shinobi who has somehow managed to control or manipulate the God of Death's powers for his own malicious purposes. This shinobi might use the power to create chaos in the ninja world, like raising the dead and making them fight against the living.
Perhaps an ancient evil that has been sealed away for centuries but has now found a way to use the concept of the Death God to break free. This entity was once a powerful ninja who turned to dark magic and now wants revenge on the modern ninja world. It could be a very powerful foe for Naruto as it has knowledge of old and forgotten jutsus.
It could be a powerful rogue ninja who wants to use Naruto's new - found 'harbinger of death' status for his own evil purposes. Maybe he believes that Naruto can lead him to powerful and dark artifacts that are only revealed during times of death and destruction.
It could be Orochimaru. Even though he's had his moments of redemption, he might relapse into his old ways. He could see Naruto's 'death' as an opportunity to carry out some evil plan that Naruto would have stopped. Maybe he wants to use Naruto's death to create chaos and then take advantage of the power vacuum in the ninja world.
It could be an evil shinobi from Naruto's world who has also ended up in the Death Parade world and is trying to corrupt the system. He wants to ensure that all the souls are sent to the void or something worse, just for his own amusement or power - hungry desires.
A possible main antagonist could be a ninja from another village who is jealous of Naruto's prankster abilities. This ninja might try to undermine Naruto's popularity in Konoha by setting up their own devious pranks that cause chaos and make it seem like Naruto is to blame.
It could be someone who is jealous of Naruto's newfound 'god of luck' powers. Maybe a rogue ninja who wants to steal that power for himself. He might constantly be plotting against Naruto, trying to find a way to take his luck away.
It could be a powerful ninja who is jealous of Naruto's new godly powers. He wants those powers for himself and will stop at nothing to get them.
Perhaps an organization that views the 'god of illusions' as a threat to their plans for world domination. They might see the 'god's' influence over illusions as something that can disrupt their schemes. So they send their most powerful agents to take on the 'god of illusions'. This could lead to some epic battles in the fanfiction.
It could be a rogue ninja who has also harnessed the power of a god of destruction. This ninja wants to use the power to rule the ninja world and Naruto has to stop him.
Maybe an ancient being that was sealed away long ago but has now been awakened due to Naruto's newfound power. This being has a natural hatred for those who can control lightning and sees Naruto as a threat to its plans of taking over the ninja world. It has the ability to manipulate shadows and uses them to counter Naruto's lightning attacks.