According to the author's website, Rowling is moving in a new literary direction.
That note reads: "Although I've enjoyed writing it just as much, my next novel will be very different from the Harry Potter Series."
So we know that, and we know that it will be published by Little Brown. And that's literally all of the information we have. So let's speculate wildly!
My money is on an erotic thriller starring a middle age man named Harold Parter who is so in love with his best friend's wife that Harold poisons him in hopes of wooing said wife during her period of mourning. It'll be called "Harold Parter And The Goblet Of Poison." What's your best guess?