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 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.
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.
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.
It could be an old rival who has also gained some sort of dark god - like powers. For example, Sasuke could have been corrupted by a powerful dark force while seeking more power, and now he stands in Naruto's way as he tries to use his new god - like abilities for good.
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.
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.
There could be a being from another dimension, a creature that feeds on chaos and destruction. It senses Naruto's new power as the Saiyan God of Destruction and wants to consume him to become even more powerful. This being would be extremely difficult to defeat as it has unique abilities that are not known in either the Naruto or Saiyan universes.
It could be an evil ninja who has also discovered the secret of the Crimson Saiyan God but wants to use it for destruction. He might be jealous of Naruto and see him as an obstacle to his plan of ruling the ninja world with this new power.