All right then. The general concept/setting is a sort of "Urban Fantasy/Sci-fi"
Long ago, a meteorite fell into a remote lake, and all sorts of exotic elements and radiation have slowly leeched out of it and into the water table- for simplicity's sake, the lake is somewhere fairly remote in the USA.
Then, a couple of hundred years ago, the area around the lake was settled, and a town was built. For a long time, everything seemed normal, until it began to dawn on folk that something strange was afoot; the local residents of the town were, every one of them, uncommonly long-lived and extraordinarily attractive, and the gender demographics were skewed heavily towards females.
And it was then discovered that effects of the meteorite had another quirk for them; that through an effort of willpower, the residents of the town were able to change genders as easily as changing clothes- So that, for instance, a high school girl might wake up one morning and decide, "I think I'll be a man today", duely become a he, and then change back to female later.
It's an idea that needs some work to fully mature, I admit. Still I thought I'd put it out there for opinions.