An organization of power - hungry individuals who are collecting powerful artifacts from different universes. Naruto and Juri cross paths with them when they are investigating the disappearance of a powerful scroll in the Naruto world. The organization sees Naruto and Juri as threats to their plans and tries to eliminate them while the two heroes strive to stop the organization from gathering all the artifacts which could lead to the destruction of multiple worlds.
One great 'naruto x juri fanfiction' story could be 'Naruto and Juri's Unexpected Alliance'. In this fic, Naruto and Juri team up to face a common enemy that threatens both their worlds. The story is filled with action-packed battles and interesting character interactions as they learn to work together despite their different backgrounds.
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 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.
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.
I think Orochimaru could also fit the bill. His experiments on himself and others are really out there. He's constantly looking for ways to gain more power and immortality in some really strange and unethical ways. His snake - like features and his ability to shed his skin like a snake add to his quirkiness. He's not your typical, straightforward villain and has a lot of idiosyncrasies that could be played up in a fanfiction.