I'm not sure exactly what this mermaid secrets' first emergency rescue story is about. Maybe it involves a mermaid in distress and some heroic act to save her. Without more details, it's hard to say for sure.
One idea could be to explore what would happen if Sora and his friends got trapped in Ariel's underwater world during a Heartless attack. They'd have to team up with Ariel and her friends to fight off the Heartless while also dealing with Ursula's schemes in a new way.
Ursula is an obvious choice as she is the main villain in the original Little Mermaid story. In a Kingdom Hearts fanfiction, she could be even more powerful, perhaps having made a deal with the Heartless or some other dark force to gain more control over the underwater world and other worlds connected to it.
One possible plot could be that Sora and his friends land in the underwater world of The Little Mermaid. Ariel could be in trouble, perhaps captured by the Heartless. Sora, with his Keyblade, would fight to free her. Donald and Goofy could help by using their magic and strength respectively. They would then have to find a way to restore the underwater kingdom's heart which has been affected by the Heartless presence.
The following are some of the system's recommendations for saving novels: Wind and Cloud Into the Painting Scroll: This is a sweet novel with an ancient setting. It tells a story of the early Tang Dynasty, full of romantic and growing elements. 2. [This novel was written by Miang. The male and female protagonists in the story…] 3. " The Bigoted Villain's Jiaojiao Pet ": The story tells the story of the female protagonist's soul transmigrating into a fictional era. She is bound to the milky white system. Her mission is to save the vicious villain and let him walk the right path in life again. 4. [Devil's Body Tempering]: The female protagonist was reborn back to 1996…These novels are all completed and can satisfy your need to save the system.
Sora would likely be very protective of Ariel. He would see her as someone in need of help, just like he does with many characters in different worlds. Ariel, on the other hand, would be fascinated by Sora's world and his Keyblade. She might ask him a lot of questions. Donald could get frustrated with Ariel's overly curious nature at times, while Goofy would just be friendly and go along with whatever is happening.