A jealous mermaid could be the antagonist. This mermaid has always been envious of the Little Mermaid's beauty and now with her unique toe - wiggling ability, the jealousy turns into hatred. She tries to spread rumors about the Little Mermaid's ability being evil and convinces some of the other sea creatures to turn against her. She also tries to sabotage the Little Mermaid whenever she tries to use her toe - wiggling power for good.
The sea witch could be the main antagonist. She might see the Little Mermaid's new - found toe - wiggling ability as a threat to her own power. Maybe she wants to steal the power for herself or stop the Little Mermaid from using it to gain more influence among the sea creatures. So she creates all sorts of obstacles and traps for the Little Mermaid.
One possible plot could be that the Little Mermaid discovers that when she wiggles her toes in a certain pattern, it creates a magical current in the ocean. This current could lead her to a hidden underwater cave filled with ancient treasures and secrets. Maybe there's a prophecy about a mermaid who can wiggle her toes in this way being the key to saving the ocean from an upcoming disaster. So she has to learn more about this special ability and face various challenges along the way.
It could be a human fisherman who has discovered a way to capture mermaids and is hunting them down for their valuable scales and tails. Aquata has to stop him before he wipes out all the mermaids in the area. Or perhaps an ancient sea monster that has been awakened from a long slumber and is causing chaos in the ocean, and Aquata is the only one brave enough to face it. This sea monster could be very difficult to defeat as it has powers that are unknown to the mermaids.
A sea witch could be a great antagonist. She might be jealous of Attina's beauty and grace, and try to use her magic to turn Attina into an ordinary fish. The sea witch could also create all sorts of obstacles for Attina and her friends, like summoning sea monsters or causing storms.
A powerful sea witch who is jealous of Marina's beauty and popularity among the mermaids could be the main antagonist. She might try to steal Marina's voice or tail to make her powerless.
A powerful sea sorcerer could be the main antagonist. This sorcerer might be jealous of the little mermaid's beauty and Octvius's unique powers. He could try to create chaos in the underwater world by stealing the magic that keeps the balance in the ocean. He might also try to turn the mermaid's friends and family against her and Octvius, using dark spells and illusions.
A human pirate could be the main antagonist. He accidentally discovers the scepter during a shipwreck and realizes its power. He then decides to use it to control the sea and all the creatures in it. The mermaids have to stop him from using the scepter in such a wrong way.
Maybe a human fisherman who is over - fishing and endangering the mermaid's home. He doesn't care about the underwater world and Mal has to stop him to protect her friends and family. So he becomes the main antagonist.
It could be an evil sea witch. She might be jealous of Lucy's newfound mermaid powers or her connection to the Fairy Tail guild. The sea witch could try to steal Lucy's powers or use her to gain access to the land world.
I think Misty from the Pokemon world could be a main character in this fanfiction. Since she has a connection with water - type Pokemon, she could easily fit into the underwater world of The Little Mermaid. Along with her, a Psyduck would be there. Psyduck's clumsy yet powerful abilities could add an interesting element to the story. And of course, Ariel would be a main character. She could learn a lot from Misty about the Pokemon world and they could team up to protect both their worlds from any threats.
Prince Eric could be a main character. He would be important as he is Ariel's love interest. Also, King Triton, Ariel's father, could play a significant role as he would be concerned about Ursula's return and want to protect his daughter and the kingdom.