Maybe a lost princess from a sunken city. She has been in hiding for a long time because of some danger that destroyed her city. She meets the little mermaid and they become friends. The lost princess has unique powers related to the ancient magic of her sunken city, and she shares these powers with the little mermaid to help her in her quests.
A sea - faring cat could be a new character. This cat has a special bond with the sailors on Prince Eric's ship. When Ariel comes on board, the cat is immediately suspicious of her. But as time goes on, they form an unlikely friendship, and the cat helps Ariel understand some of the human ways that the sailors have.
A merman who was once in love with Ariel but turned bitter when she chose Eric. He could team up with some sea monsters and try to take Ariel back to the sea against her will, believing that she made the wrong choice in leaving the ocean for a human.
A group of mermaids from a far - off and isolated reef who are jealous of Ariel's family and their influence. They could plot to make Melody look bad in front of the underwater kingdom.
A wise old sea turtle could be a new character. He could be the one who gives the mermaid advice about the human world and warns her of the potential dangers.
There could be a rival mermaid who is jealous of the main mermaid's transformation. This rival might try to sabotage the transformation or create trouble for the mermaid in her new human life. She could be envious because she always wanted to experience the human world but was too afraid to take the risk of transformation.
I think Misty from the Pokemon world could be a main character in this fanfiction. Since she has a connection with water - type Pokemon, she could easily fit into the underwater world of The Little Mermaid. Along with her, a Psyduck would be there. Psyduck's clumsy yet powerful abilities could add an interesting element to the story. And of course, Ariel would be a main character. She could learn a lot from Misty about the Pokemon world and they could team up to protect both their worlds from any threats.
A sea turtle that has seen many centuries pass could befriend Melody. The sea turtle could tell her stories about the ancient times of the ocean, give her advice on how to handle difficult situations, and guide her through some of the more dangerous areas of the ocean that are filled with magic and mystery.
A merman from a far - off part of the ocean. He could have unique powers, like the ability to control the tides. He might befriend Ariel and join her on her adventures, or he could be a rival for her affections.
Prince Eric could be a main character. He would be important as he is Ariel's love interest. Also, King Triton, Ariel's father, could play a significant role as he would be concerned about Ursula's return and want to protect his daughter and the kingdom.
If it's a crossover with 'Frozen', Elsa and Anna would be great characters. Ariel could meet them when a magical iceberg drifts into her underwater world. Elsa's ice powers and Ariel's water powers could interact in interesting ways.