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.
A new villain created specifically for the crossover could be really interesting. This villain could be a powerful entity that has the ability to manipulate the energy from both the Naruto universe (like chakra) and the Mario universe (like the power - ups). He wants to merge the two worlds into a chaotic realm where he can rule supreme, forcing Naruto and Mario to put aside their differences and fight together.
Adventure is also a big theme. The characters embark on new quests in unfamiliar settings. For instance, if Naruto is in the Mushroom Kingdom, he has to figure out how to deal with Goombas and Koopas while still maintaining his ninja skills. And for Mario in the Hidden Leaf, he has to adapt to the more dangerous and stealth - based ninja world. There's also the theme of power, as they combine their unique powers to face challenges.
A great choice for the main villain could be a fusion of Bowser and a powerful alien from another game. This new villain could have Bowser's brute strength and the alien's advanced technology. They could be bent on taking over both the Mario universe and the other game's universe, using the power of the galaxies to fuel their evil plans.
Bowser and King Dedede teaming up would be really interesting. Bowser's brute strength and his ability to use fire combined with King Dedede's hammer - swinging skills and his own army of minions would pose a great threat to Mario and Kirby. They could be trying to take over both the Mushroom Kingdom and Dream Land for their own greedy purposes, like hoarding all the power - ups and food in both worlds.
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.
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.
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.
A great plot could involve a new threat that is a combination of the dark forces from both universes. Naruto and Mario find themselves in a strange new dimension where the rules of both their worlds blend. They must learn to trust each other and combine their unique skills. For example, Mario's invincibility star could enhance Naruto's speed and strength during battles, and Naruto's sage mode could help him sense hidden traps in the Mushroom Kingdom like he does in his own world.