Another option could be a group of rogue robots who have their own set of rules and don't like the idea of Bender and the Robot Devil being so prominent. They could band together to try and take over the areas that Bender and the Robot Devil operate in. They might be more brutal and less interested in the games and deals that the Robot Devil usually plays, making them a very different kind of antagonist compared to the Robot Devil.
One idea could be that Bender and the Robot Devil get into a competition to see who can steal the most valuable items in the universe. Another could be that they team up to prank all the other robots in town. Maybe they start a robot - only casino together as well.
It could be a rogue ninja who has mastered the devil breakers in a very dark way. He might use them to gain power and control over the ninja world, seeing Naruto as the main obstacle to his plans.
It could be a powerful demon lord who wants to conquer both the Naruto universe and the Devil May Cry world. This demon lord might have the ability to manipulate chakra and also use demonic powers, making it a formidable foe for both Naruto and Dante.
It could be a rogue god who sees the half - robot as a threat to the natural order. They might try to destroy it because they don't like the idea of a half - human, half - machine being in the world of demigods. Another possibility is a powerful Titan who was awakened and wants to use the half - robot's technology for their own evil purposes. Maybe the half - robot has some sort of power source that the Titan desires.
It could be a powerful devil who sees Naruto as a threat to his rule in the devil realm. Maybe this devil wants to use Naruto's power for his own evil purposes and Naruto has to resist.
It could be a powerful devil who has the ability to control minds. This devil could manipulate some of Naruto's friends and turn them against him, creating a lot of internal conflict within Konoha.
It could be a young and ambitious devil who has grown tired of the old ways of the underworld. He might see the current power - holders as corrupt and decides to take matters into his own hands by declaring war.
It could also be a human scientist who has been experimenting with demonic energy. He becomes corrupted by this power and decides that he should control both the human and demon worlds. He sees Devilman and Devil Lady as obstacles in his path and uses his scientific knowledge and newfound demonic powers to try and defeat them.
It could be a new character created specifically for the fanfic. Maybe a powerful and malevolent spirit that has been lurking in the shadows of the Naruto world.
It could be Voldemort still. Since Harry has awakened the devil gene, Voldemort might see this as an opportunity to either corrupt Harry further or use his new power for his own dark purposes. So he would be the main force against Harry.