Their baby could look very much like a normal human baby at first glance, but on closer inspection, it has some mermaid - like qualities. For example, its ears might be slightly pointed like Ariel's, and when it's in the water, it seems to move with an unusual grace. It has a beautiful face with a combination of Ariel and Eric's best features, and its skin has a soft, almost iridescent sheen in certain lights.
Maybe the plot involves the baby getting kidnapped by an evil sea witch who wants to use the baby's unique heritage for her own dark purposes. Ariel and Eric then embark on a dangerous adventure to rescue their baby. Along the way, they meet various sea creatures and humans who either help or hinder them. In the end, they manage to save their baby through their combined strength and love.
Well, it could have a body shape that's a mix of Greninja's sleek and athletic build and Delphox's more elegant and slender form. Its head might be more like Greninja's with a sort of frog - like appearance, but with Delphox's fiery markings around its eyes. It may also have a tail that combines the features of Greninja's small tail and Delphox's large, bushy tail, perhaps with some sort of water - fire elemental design on it.
A human noble who doesn't approve of Ariel and Eric's union and especially doesn't like the idea of a half - human, half - mermaid baby. This noble could try to create problems for the family, like spreading false rumors or trying to get the baby separated from its parents. Another possibility is a sea monster that has been awakened by the birth of the baby, seeing it as an intrusion in its territory and trying to harm the baby and its parents.
The baby might have Bella's big, expressive eyes and Damon's dark, tousled hair. Maybe it would inherit Bella's pale complexion and Damon's sharp facial features. Since they both have a certain allure in their respective shows, the baby could be extremely beautiful or handsome, with an otherworldly charm.
One idea could be that Ariel and Eric are taking their baby on a magical underwater tour to introduce it to all of Ariel's friends in Atlantica. Another could be the baby showing signs of having both human and mermaid/merman powers and Ariel and Eric trying to figure out how to help the baby control them. Maybe there's a plot where an old enemy of Ariel's resurfaces and they have to protect their baby from this threat.
She could have Katniss's dark hair and Peeta's blue eyes. Her features might be a blend of their strong and determined looks. Maybe she has Katniss's high cheekbones and Peeta's soft - looking lips.
In fanfiction, the baby could inherit Sesshomaru's grace and strength. It could have a lithe body like him. However, it would also have Kagome's human charm. Its eyes could be a mixture of Sesshomaru's intense gold and Kagome's brown, creating a really unique and alluring look. And perhaps it would have some sort of special power that combines Sesshomaru's demon abilities, such as his speed and regeneration, with Kagome's spiritual powers.
The baby could be a really interesting mix. It might have Hermione's bushy hair and Legolas' pointed ears. And perhaps inherit Hermione's intelligence and love for books as well as Legolas' amazing archery skills and connection to nature.
The baby could be a really interesting mix. Maybe it would have Finn's human features like his hair color and some of his facial structure, but also have the candy - like features of Princess Bubblegum. So it could have pinkish skin or some sort of candy - themed markings on its body.
Perhaps the baby would have long hair like Fuu, but blond like Naruto. Its eyes would be a bright blue, with a glint of mischief similar to Naruto's. The baby's skin tone could be a bit lighter than Naruto's, more towards Fuu's complexion. And it would have a cute little nose that twitched when it was excited, just like Fuu's nose does when she's curious.