It could be a rogue ninja who has somehow learned about Thea Queen's world and wants to conquer it using his ninja techniques. He might see it as a new land full of resources and weak opponents compared to the shinobi world.
Naruto might be very energetic and friendly towards Thea Queen Lemon. He would probably greet her with his big smile and introduce himself. Thea Queen Lemon could be initially a bit reserved but Naruto's openness would gradually make her more comfortable.
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.
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.
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 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.
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 an evil wizard who wants to use the power of the ninja world and the wackiness of the Looney Tunes world for his own dark purposes. He might try to capture Bugs Bunny to use his unique abilities and Naruto to harness his chakra. This wizard could be creating chaos by merging parts of the two worlds in a harmful way.
Another plot could involve Naruto visiting Thea Queen's world. He might be confused by the advanced technology at first but quickly adapts. Naruto could use his shadow clone jutsu to help Thea Queen in a fight against her enemies in Star City. They could also learn from each other's sense of justice and determination.
In this fanfiction, it could be that Thea Queen Lemon has a special artifact that is coveted by many villains. Naruto stumbles upon her while she is on the run from these bad guys. At first, they don't trust each other. But as they are forced to fight together, they start to respect and rely on one another. The story could then focus on their journey to find a safe place to hide the artifact and along the way, they face many challenges that test their newfound friendship and their individual strengths.