A powerful demon lord from the underworld could be the main villain. This demon lord could be seeking to expand his territory into the Naruto world, using his dark magic and hordes of minions.
It could be a primordial being from the underworld that was sealed away long ago but has now been released. This being is angry at the world of Naruto for some ancient grudge and wants to bring destruction. It has the ability to control the minds of the weak - willed and uses this to create chaos in the Naruto world as a first step towards total annihilation.
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.
It could be Orochimaru. Imagine he discovers the power of the ocean in Moana's world and wants to use it for his own dark experiments. He could start corrupting the sea creatures and causing chaos on Moana's island. Naruto, having faced Orochimaru before, would know his weaknesses and team up with Moana to stop him.
It could be Bowser who has somehow gained the power of a powerful ninja jutsu. He uses this new - found power to take over both the Mushroom Kingdom and start invading the Naruto world. His fire - breathing ability combined with ninja techniques would make him a formidable foe for Naruto and Mario.
Kaguya could be a great main villain for this. Imagine Kaguya emerging on Halloween night, her already otherworldly appearance made even more menacing with spooky Halloween - like enhancements. She could use the energy of the night to her advantage, trying to plunge the world into eternal darkness, and Naruto and his friends have to stop her.
Perhaps it could be a character who is jealous of both Naruto and Hercules. A sorcerer who was once shunned in both worlds and now wants to prove himself by defeating them. He could use dark magic to create chaos and try to stop Naruto and Hercules from uniting and using their combined powers.
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.
Obviously Naruto himself would be a main character. He could be joined by Sasuke, who might have his own reasons for being interested in the underworld. For the 'lemon' aspect, Sakura could be involved as a love interest for Naruto. Also, characters from the underworld like a powerful underworld lord or a female warrior who catches Naruto's eye could be main characters.
It could be Orochimaru. He might be interested in Tenten's unique weapon skills and Naruto's power of the Nine - Tails. He could try to capture them to conduct experiments on how to combine their powers for his own gain. He might create some sort of hybrid - clone with Tenten's weapon - using abilities and Naruto's chakra.
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.
Maybe a new character, a powerful sea witch who has been influenced by the power - hungry ways of some of Naruto's villains. She wants to rule both the underwater world and the land world above. She would use her magic to create chaos and try to stop Naruto and Ariel from uniting their forces.