Percy Jackson is Poseidon's son, God of the Sea, The Little Mermaid, Ariel is Triton's daughter, king of the sea and Poseidon's son, Aquaman is the prince of the sea, which makes him Triton's son. Based on this, Percy Jackson could be Triton's half brother, and Ariel and Aquaman could be siblings.
In Once Upon a Time they show Ursula as Poseidon's daugher; what if this is true? This would make her Ariel's and Triton's evil and jealous aunt.
We know from Greek mythology Heracles (Hercules) as Zeus' son, in Percy Jackson and the sea of Monsters we have Thalia, Zeus' daughter; this could make her and Hercules half siblings?
This is my theory of what would the story be like if all the characters were connected to one another, hope you liked it.