I've got one, but if you use it I want royalties:
There's a woman... and she gets dumped by her boyfriend... and in a rage she shouts "I hate my life! I wish I was a fish!", you know, as you do when you're in a rage, you say "I wish I was a fish!", well she did. Anyway, for some reason that wish comes true, and she becomes a fish, and she's a fish for a while, she's in a pond for a bit, and then she turns back into a human... but she still has a fish brain! So now, she's a human with a fish brain, and she has to live her life as a fish in humans skin. Kinda like a psychological twist on Jonah and the whale.
I think the moral is quite powerful... don't wish you're a fish, because if you do... you might become a fish...
It should either be called 'The Woman Who Wished She Was a Fish and Then Was a Fish (and didn't like it)', or 'To Wish to be a Fish is a Foolish Wish because a Fish Should be a Fish not a Human'.
Obviously it's pretty open ended so there's space for a sequel. Dunno, maybe she gets addicted to bait or something in the next film...