Here's another plot for The Little Mermaid III:
It could take part after the second movie, but all of the heroes would have the same age as in the second movie, not older.
King Triton has a son, but very few know about it. His name is Apollo and he was taken away from the kingdom at the age of 3, because he was the only male-heir to the throne and Triton wanted to protect him from Ursula. He would grow up with nannies in a far away kingdom.(here we could settle everything about Ariel's past including her mom, as a flash back). Apollo returns to Atlantica to be king because Triton eants to retire. This fact Brings Ariel, Eric and Melody to Atlantica(as merpeople of course).
There is also Marilyn, Ursula's beautiful and kind daughter, and Octavius, her revengeful brother who wants...GUESS!!! to take over Atlantica!
Marilyn will meet Apollo by luck and they will fall in love, but Octavius will use his sister to get what he wants.After that Apollo'll feel betrayed, but Ariel will explain everything to him, and all together, they will have to save Atlantica from Octavius...
Now I KNOW what you're going to say(isn't it Lion King II plot? etc.etc.)
It's just an idea I'm trying to improve...
