Often, Eric is portrayed as a brave and curious character. He's not afraid to explore the unknown, which is how he ended up encountering Ariel in the first place. His love for the sea and his adventurous spirit make him an interesting figure in fanfiction.
Often, Ursula is depicted as a more nuanced character. She could be seen as a victim of circumstance in some fanfics.
Perhaps in the fanfiction, Ariel and Eric get married and have to deal with the cultural differences between the underwater world and the human world. For example, Ariel has to learn the proper etiquette in Eric's kingdom while Eric tries to understand the unique traditions of Ariel's mermaid family.
Perhaps in a fanfiction, Ariel and Eric will have a family of their own. They will raise their children to understand both the land and the sea, creating a unique bond between the two worlds.
One possible fanfiction could be that Ariel and Eric discover a hidden underwater kingdom that was lost long ago. They embark on a journey to learn about its history and find that it holds the key to a magical power that can save both the underwater and land worlds from a new threat.
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.
She could be portrayed as a rebel. Instead of always following the rules of the mermaid kingdom, she might start to question them and lead a movement for change among the mermaids.
Often, in fanfiction, Ursula can still be a main antagonist. She might come back with a new plan to break up Ariel and Eric, maybe using a different form of magic or disguise this time.
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.
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.
A great storyline could involve Eric's adventures before he met Ariel. Maybe he was on a quest to find a mysterious island, and during that journey, he had some premonitions or signs that led him to Ariel later. It could also be about Eric dealing with the political pressure in his kingdom regarding his marriage to Ariel, as she is from a different species.