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 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 an evil Saiyan who comes to the Naruto world to cause chaos. Maybe he's a rogue Saiyan who was exiled from his own planet and wants to take over Naruto's world.
Well, it could be about Naruto somehow obtaining the powers of a Saiyan God of Destruction. Maybe he's sent to a different dimension where he has to learn to control this new, incredibly powerful form. He might face off against powerful foes that only this new power can defeat.
It could be a powerful ninja who has made a deal with an otherworldly force to gain powers to match Naruto's. This antagonist might be jealous of Naruto's unique heritage and wants to prove that pure ninja power is superior. So, he uses dark magic or forbidden jutsu to become a formidable foe.
It could be an evil ninja who has already made a pact with the Abyssal God and wants to use its power to take over the ninja world. He might see Naruto as a threat to his plans because Naruto has a natural connection to the god's power.
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.
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 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.
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.