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 is also a master of illusions and wants to use his powers to take over the ninja world. He might see Naruto as a threat to his plans since Naruto is developing his illusion skills.
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.
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.
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.
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.
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.
Well, in this fanfiction, perhaps a really cool concept is the origin story of the 'god of illusions'. It could be that this entity has a deep connection to an ancient and forgotten clan in the Naruto world. Also, the idea of the 'god of illusions' being able to manipulate not just what people see, but also their emotions and memories through illusions is quite fascinating.