Who could be the main antagonist in 'little mermaid legend of octvius fanfiction'?

In the fanfiction, Octvius could be a shape - shifting creature. He appears to the little mermaid in different forms, sometimes as a dolphin to play with her, other times as a mysterious merman with special powers. The little mermaid is intrigued by him and tries to find out his true identity. As they get closer, they find out that Octvius is a key to unlocking a hidden underwater realm filled with ancient treasures and secrets. But there are also guardians of this realm who will stop at nothing to keep it hidden, so they have to fight together to explore it.

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.

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 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.

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.

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.

Maybe an enemy spy who has infiltrated the little soldier's unit. This spy would be constantly trying to undermine the unit's operations and would be a constant threat to the little soldier and his comrades. He could be very sneaky and hard to detect, which would make him a formidable antagonist.