Perhaps the main villain could be a new character created specifically for this crossover. This character could be a powerful ninja - like being who has somehow gained god - like powers from the God of War world. He might have a grudge against both Naruto and Kratos. For example, he could be a former ninja who was exiled from Naruto's world and found a way to gain power in the God of War world. Now he wants revenge on Naruto for his past and also wants to conquer the God of War world, seeing Kratos as a threat to his plans.
In this crossover, the main villain might be a combined form of Orochimaru from Naruto's world and Ares from God of War. Orochimaru's thirst for power and Ares' love for war could create a really evil and power - hungry entity. This villain could be trying to take over both the ninja world and the God of War realm. He would use his knowledge of ninjutsu and godly powers to cause chaos. Naruto and Kratos would have to put aside their differences and fight this powerful foe together. Kratos' brute strength and Naruto's strategic thinking would be essential in taking on this complex villain.
It could be a combined villain. For example, Orochimaru from the Naruto universe teams up with a powerful magic - user from Wanda's world who is jealous of Wanda's powers. This villain duo could be really menacing as they combine Orochimaru's dark jutsu and the magic - user's powerful spells.
It could be Orochimaru from Naruto teaming up with Dr. Eggman. Orochimaru is always seeking power and new forms, and Dr. Eggman wants to take over the world. Together, they could create some really dangerous machines and jutsu combinations to try and achieve their goals.
It could be Orochimaru from Naruto who somehow finds his way into the Lion King world. He would be interested in the unique magic and power of the Pridelands. He could try to control the lions or use them for his evil experiments, like he did with his human subjects in Naruto.
Perhaps a corrupted version of one of Naruto's comrades who has been influenced by the dark powers of Lucifer's world. This character would know all of Naruto's weaknesses and use them against him. At the same time, they would also have new demonic abilities, making them a formidable foe for both Naruto and Lucifer.
The main villain could be a rogue ninja who has somehow accessed the power of the Darksiders world. He has turned evil and wants to reshape both worlds according to his own twisted vision. He uses his ninja skills combined with the dark powers he has acquired to create an army of minions from both universes. Naruto and Fury must put aside their differences and use their combined skills to take on this powerful and unpredictable foe.
It could be Loki. In the Norse mythology, Loki is a trickster and a very powerful being. He might be jealous of Atreus' growing power and see him as a threat. So, he devises elaborate plans to either turn Atreus against Kratos or directly attack them both.
Perhaps an evil Pokemon scientist who has created a device that can control the minds of Pokemon. He discovers Naruto's arrival and sees him as a threat. So, he teams up with some of the most powerful and aggressive Pokemon to stop Naruto. He has no regard for the well - being of Pokemon or the balance of the world, only his own ambition to rule.
It could be a powerful demon that has the ability to absorb the powers of others. This demon has been observing both Gohan and Naruto from the shadows and wants to take their powers to become invincible.
It could be an evil ninja who has learned to use dark magic and wants to take over Pixie Hollow for its magic. He sees the fairies' magic as a source of power to enhance his own dark jutsu.
Another option could be Kabuto. He has been influenced by Orochimaru and is always looking for ways to gain more power. He might target Kurenai to learn more about her genjutsu and Naruto because of his status as a hero. Kabuto could be plotting to use their powers to further his own goals in the ninja world.