One possibility is Kabuto. He could be working with some rogue underwater creatures. He uses his medical ninjutsu to create mutant sea creatures that are out of control and attack both the mermaids and any humans that come near the water. His goal could be to find some ancient power hidden in the ocean that he believes will make him invincible.
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.
Ursula is an obvious choice as she is the main villain in the original Little Mermaid story. In a Kingdom Hearts fanfiction, she could be even more powerful, perhaps having made a deal with the Heartless or some other dark force to gain more control over the underwater world and other worlds connected to it.
A merman who was once in love with Ariel but turned bitter when she chose Eric. He could team up with some sea monsters and try to take Ariel back to the sea against her will, believing that she made the wrong choice in leaving the ocean for a human.
A group of mermaids from a far - off and isolated reef who are jealous of Ariel's family and their influence. They could plot to make Melody look bad in front of the underwater kingdom.
Well, a Naruto and Little Mermaid fanfiction could work in several ways. One idea is that Naruto could be transported to the underwater world of the Little Mermaid. He might use his ninja skills to adapt, like using his chakra to swim faster or create air bubbles to breathe. Maybe he could even teach the mermaids some ninja techniques in return for their help in finding a way back home.
In a Naruto and the Little Mermaid crossover, Naruto could be transported to the underwater kingdom. There, he meets Ariel who is fascinated by his strange powers. Together, they could team up to find a way to stop an evil force that is threatening both the surface world (where Naruto is from) and the underwater world. Naruto's ability to make clones could be used in interesting ways underwater, like creating distractions for the bad guys.
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.
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.
In a reverse little mermaid story, the mermaid herself could be the villain. Instead of being the kind - hearted character we know, she could be cruel and use her charm to manipulate humans for her own gain.
If it's a crossover with 'Frozen', Elsa and Anna would be great characters. Ariel could meet them when a magical iceberg drifts into her underwater world. Elsa's ice powers and Ariel's water powers could interact in interesting ways.