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.
The main characters are, of course, Ariel and Baby Melody. Ariel is a mermaid known for her curiosity about the human world. Baby Melody is Ariel's daughter, who has her own charm and inquisitiveness.
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.
Perhaps a plot where Baby Melody gets lost, and Ariel has to search for her all over the ocean. Along the way, Ariel meets different friends and foes, and through her determination, she finally finds Melody. It could show the strong bond between them.
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.
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.
A possible fanfiction is that they time - travel back to an important historical event. Blake and Baby Ariel have to use their modern - day knowledge and skills to navigate through the past and accidentally change history in a positive way.
Another way is to follow fanfiction authors who are known for writing about similar celebrities or topics. They might also write some interesting Blake Gray and Baby Ariel fanfiction.
I'm not sure who Ariel is in this context without more information. It could be a unique character created specifically for this 'never - ending story' that has its own set of characteristics and a role within the plot.