I read/saw somewhere that at one point in the Little Mermaid movie, Ursula was going to be Ariel's estranged aunt.

And at one point in the song Poor Unfortunate Souls, Ursula says that magic is a gift she's always had (or something along those lines; I'm too lazy to look it up). It would make sense for magic to be an inherited trait, so why wouldn't Ariel have a gift for magic too in this AU where Ursula is her aunt?

So I was thinking what if when Ursula is telling Ariel to give up her voice, instead if doing it Ariel comes to the same conclusion I did and is like, "I don't really want to lose my voice... what if you taught me magic and I could give myself legs eventually?"

And of course at first Ursula's totally against it; why would she want an apprentice? But then she realizes that it would actually be great revenge on her brother (King Triton) to make his youngest daughter into another Sea Witch.

So a couple years go by and Ariel is really good at magic, she still sees her sisters occasionally but never her father. Ariel finds out that Ursula isn't as bad as she pretends to be, and Ariel is a good influence on the Sea Witch.

And when Ariel gets good enough at magic to give herself legs, Ursula realizes she will miss her niece. But Ariel has decided that she will stay on land; Prince Eric was just a crush anyways, and besides, now she has magic.

And eventually Ariel convinces Ursula to reunite with Triton and she ends up becoming that weird aunt.

The end!

If anyone knows of something like this or wants to write it, then please let me know!