One possible plotline could be Vanessa's backstory. Maybe she was once a kind - hearted woman who was corrupted by dark magic. Another could be a redemption arc, where she tries to make amends for her actions against Ariel. For example, she might help Ariel in a difficult situation, like facing a new threat in the ocean world.
Perhaps a plotline where Vanessa is actually an enchanted mermaid herself. She was cursed to become a human and steal the prince away from Ariel as part of a revenge plot by an evil sea witch against Ariel's father. But then she starts to feel guilty and teams up with Ariel to break the curse on both of them.
One possible plot could be that Eric, after being under Vanessa's spell briefly, has hazy memories of Ariel. Vanessa tries to keep Eric away from the sea, but Eric starts having dreams of a mermaid. As he investigates, he discovers Vanessa's true nature. He then sets out to find the real girl he saw in his dreams, leading him back to Ariel.
Ariel could die due to a tragic accident. For example, she might be exploring a sunken ship when it collapses on her. Unable to escape in time, she gets trapped and runs out of air. This sudden and unexpected death would shock those who knew her, both in the underwater realm and among the humans she had befriended. It would also lead to a great deal of mourning and could potentially drive Prince Eric to seek revenge on those he believes are responsible, like pirates who might have looted the ship and weakened its structure.
Well, one idea for this fanfiction could be that Vanessa, in her quest to keep Eric away from Ariel, tries to make him believe that mermaids are evil. She weaves tales of sea monsters and how mermaids are their allies. However, Eric has his doubts. He has seen Ariel's pure heart and can't quite believe Vanessa's stories. As he starts to investigate, he uncovers Vanessa's true identity and her devious plans.
A possible plotline could be that Cam and Elizabeth are humans who accidentally discover a magic portal to the underwater world of The Little Mermaid. Once there, they have to adapt to living underwater while also trying to find a way back home. Along the way, they form friendships with the mermaids and sea creatures and get involved in a conflict between Ursula's minions and Ariel's family.
In this fanfiction, Vanessa might have a change of heart. She realizes that her actions are wrong and decides to help Eric find Ariel. Along the way, they share many adventures and grow closer. Eric begins to see Vanessa as a friend, and she has to suppress her growing feelings for him. Their relationship is filled with tension as Vanessa battles her own emotions while trying to do the right thing.
The relationship could develop in a negative way. Vanessa might start to be more controlling over Eric as she realizes he is starting to have doubts. She could try to use more powerful magic to keep him in line.
One way to develop Vanessa is to make her a more complex villain. Instead of just being a tool of the sea witch, she could have her own plans and desires. Maybe she wants to rule the underwater world herself. This would create more conflict and also give her character more depth. We could also explore her relationship with other characters in the story, such as the prince's family or the servants in the palace.
One possible plotline could be little Thranduil learning important lessons from his father Oropher about leadership. For example, Oropher might take Thranduil on a hunting trip in the forest and teach him how to read the signs of the land, which is a valuable skill for a future leader. Another could be Thranduil getting into trouble while exploring on his own and Oropher having to rescue him, which would strengthen their bond. Or perhaps a story where Oropher tells Thranduil tales of their ancestors, passing on their family's history and values.
In one alternate ending, Ariel might not choose to be with Prince Eric. Instead, she realizes that her true place is in the sea with her family and friends. So she gives up on the idea of becoming human permanently and returns to her mermaid form, ruling the underwater kingdom beside her father.
Maybe in the fanfiction, it turns out that the mermaids in 'The Little Mermaid' are actually part of an ancient prophecy related to 'Gravitation'. For example, they are the key to saving the world in 'Gravitation' from some sort of musical apocalypse. This would turn the whole story on its head as the characters from 'Gravitation' would have to rely on the mermaids and their unique abilities, which are completely different from what they are used to in their own world.