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.
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.
Another interesting concept is an OC who is a member of an organization that opposes Organization XIII in a different way. Maybe they use magic that is not based on darkness or light, but something in between, which causes them to be shunned by both sides at first.
One idea could be exploring the backstories of the new characters introduced in Kingdom Hearts 3 in more depth. For example, you could write about how they came to be in the different worlds and what their true motives are.
One idea could be a fanfiction where the characters from Kingdom Hearts watch their own adventures from an alternate universe. It would be really cool to see their reactions to different versions of themselves and the events that unfold.
One idea could be a crossover where Sora and his friends from Kingdom Hearts end up in the Pokemon world. They could team up with trainers to battle against the Heartless that have somehow invaded the Pokemon realm. Another idea is that some Pokemon could have the power of Keyblades, and they use it to protect their regions from both Pokemon villains and the threats from Kingdom Hearts like Organization XIII. Maybe a Pokemon like Pikachu could befriend Sora and they go on an adventure together to find a way to get Sora back to his own world while also helping the Pokemon world.
One idea could be a story where the Pokemon trainers are transported to the Kingdom Hearts universe and have to team up with Sora and his friends to fight against the Heartless. They could use their Pokemon's unique abilities in combination with keyblades to defeat the enemies.
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.
A cool fanfiction idea might be to set the story in a modern - day world. Ariel could be a mermaid who discovers a portal to our world and falls in love with a surfer named Eric. There could be a lot of fun cultural clashes as she tries to understand things like cell phones, cars, and modern fashion. Also, Ursula could be a powerful businesswoman who tries to stop Ariel from being with Eric by ruining his surfing career.
Well, in a 'The Little Mermaid 2023 Fanfiction', you could write about Ariel's life after she becomes human. Maybe she struggles to fit in not just with the humans but also with the mermaid society when she tries to bring about some changes. You could also focus on the relationship between Ariel and her sisters. What if they had different reactions to her becoming human? One sister could be jealous while another could be overly protective. This would add a lot of drama and new plotlines to the fanfiction.
One idea could be exploring what would happen if the Little Mermaid had a different power. For example, instead of just having a beautiful voice, she could have the power to control the tides. This would change the dynamic of her relationship with the other sea creatures and also how she approaches her adventure on land.