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.
One idea could be that Devilman and Devil Lady team up to fight a common enemy that is threatening both their worlds. Maybe a powerful demon from another dimension is trying to take over, and they have to combine their powers to stop it.
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 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.
If it's a crossover with Alice in Wonderland, the Queen of Hearts could be the main antagonist. The Queen might see Tinkerbell as an intruder and try to behead her. Tinkerbell would have to use her wits and magic to survive in the crazy world ruled by the Queen of Hearts.
Morgana could be a great antagonist. She might see the Lady of the Lake as a threat to her own power and plans for Camelot. And she could try to stop any alliance between Merlin and the Lady of the Lake.
It's hard to say for sure, but Dante has a lot of experience and a wide array of powerful weapons and abilities. He has his Rebellion sword, Ebony and Ivory guns, and his devil trigger form which gives him enhanced strength, speed, and durability. However, Devilman also has his own unique powers, being part - human and part - devil. His ability to blend in with humans while having devil - like strength could give him an edge in certain situations.
A powerful demon from another dimension could be the main antagonist. This demon could be attracted to the power that Sinbad wields in the Magi world and wants to steal it for himself. He could start causing chaos in both the world Sinbad is from and the world he's been transported to in the crossover.
A new character could be introduced as the main antagonist. For example, a being from another dimension who has been observing both Sonic's and Megaman's worlds. This being realizes that by combining the chaos energy from Sonic's world and the advanced robotics technology from Megaman's world, it can create a new, all - powerful empire. So it starts to manipulate events in both worlds to bring about this combination, while Sonic and Megaman have to stop it.
Dr. Eggman and Dr. Wily could team up. Dr. Eggman has his mechanical creations and knowledge of Sonic's world, while Dr. Wily is an expert in creating powerful robots in Mega Man's world. Together, they could be a formidable force against Sonic and Mega Man.