One interesting storyline could be Ariel and Eric exploring new lands together that are beyond the sea and the shore they know. They could discover hidden islands with magical creatures and have adventures while learning more about each other's worlds.
A possible storyline is that Ariel and Eric start a school in their kingdom where they teach the land - dwellers about the sea and the mermaids teach the mermaids about the land. This fanfiction could focus on how they bridge the gap between the two worlds and how their relationship grows even stronger as they work together towards this common goal.
Perhaps a fanfiction where Ariel and Eric have to overcome a curse that has been placed on their relationship. This curse could be the work of an evil sea witch who is jealous of their love. They would then have to go on a quest to break the curse, facing many challenges along the way. This would involve Ariel using her mermaid powers and Eric his skills as a prince to fight against the forces trying to keep them apart.
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.
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.
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.
One possible storyline could be that Ariel's sisters discover a hidden underwater kingdom that was lost long ago. They embark on a journey to explore it and learn about its history and secrets.
In the fanfiction, a romantic plot could involve Ariel getting the ability to walk on land permanently. Eric then takes her on a tour of his kingdom, showing her all the beautiful places. At sunset, he takes her to a cliff overlooking the ocean and proposes to her, making it a very romantic scene.
One possibility is that they immediately start planning their future. Since they come from two different worlds, they'd have to figure out how to make their relationship work. Maybe they start building a place where both of them can be comfortable, half on land and half in the sea. And they could start spreading the message of love and unity between humans and mer - people.
Another idea is to write a fanfiction where Ariel has to deal with a new threat to the ocean. It could be an evil sea witch who is more powerful than Ursula. Ariel would have to use her wits and the help of her friends to save the ocean and all its inhabitants.
One idea could be that Ursula has a change of heart after Ariel saves her from a dangerous sea creature. Maybe Ariel gets lost in a part of the ocean Ursula controls, and Ursula decides to help her find her way back instead of taking advantage of the situation.