One possible plotline could be that Ariel and the urchin are from different worlds and they have to overcome various obstacles to be together. Maybe there are cultural differences between their species that they need to navigate.
I'm not sure of specific 'ariel x urchin fanfic' off - hand. But you can try searching on fanfiction websites like Archive of Our Own. They usually have a wide variety of fan - created stories that might include what you're looking for.
It's not appropriate at all. Such content is not suitable for public consumption and goes against the general norms of decency. Moreover, it may also be in violation of the original creators' intentions for the character of Ariel.
One possible plot could be that Ursuala and Ariel become friends despite their differences. Maybe Ursuala, known for her dark magic, uses her powers to help Ariel when she gets into trouble in the ocean that normal mermaid abilities can't handle. For example, a new kind of sea monster threatens the mermaid kingdom, and Ursuala's magic is the key to defeating it.
One common theme is exploration. Since Sora is used to traveling between different worlds in his own story, and Ariel has a sense of adventure in exploring the human world, their fanfics often involve them exploring new places together.
Imagine that a dark force is spreading across the ocean, making the waters murky and the sea creatures sick. Moana notices this on her travels and Ariel also experiences it in her underwater world. They both independently set out to find a solution. By chance, they cross paths. At first, they have some misunderstandings due to their different ways of life - Moana is a human used to the surface world and Ariel is a mermaid from the underwater world. But they soon realize that they need to work together. They follow a series of clues left by an ancient sea spirit. Along the way, they learn about each other's families and the unique traditions of their homes. Finally, they find the way to defeat the dark force and in doing so, bring new hope and harmony to the ocean.
Ariel could be any character created by the fanfic author. It might be a mermaid, as the name Ariel is often associated with mermaids in popular culture.
To develop the urchin character, you could give it a distinct personality. It could be a bit of a trickster, playing small pranks on other sea animals but always having a good heart. Also, you can explore its relationship with its family or the urchin community. Maybe it's an outcast among them because of its friendship with the mermaid, which adds depth to its character.
You could write a fanfic about Ariel's adventures in the human world after she becomes human permanently. She could be exploring new cultures, making new friends, and learning about human history and art. This would focus on her growth and discovery in a new environment.
Well, it's hard to say it's a widely popular genre. There are so many different types of fanfic out there. Some people might be really into this kind of unique pairing between Ursula and Ariel, but it probably doesn't have the mass appeal that more common pairings in Disney fanfic might have. A lot of factors like the overall perception of the characters and the themes that can be explored in such a pairing play a role in its popularity.