It's starts with the Harry Potter tie-in book, Fantastic Beasts and Where to Find Them. The film will follow a character named, Newt Scamander, who came before Harry. We will see some new folks and a young Albus Dumbledore. From the press release, J.K. said:
It all started when Warner Bros. came to me with the suggestion of turning Fantastic Beasts and Where to Find Them into a film. I thought it was a fun idea, but the idea of seeing Newt Scamander, the supposed author of Fantastic Beasts, realized by another writer was difficult. Having lived for so long in my fictional universe, I feel very protective of it and I already knew a lot about Newt. As hard-core Harry Potter fans will know, I liked him so much that I even married his grandson, Rolf, to one of my favorite characters from the Harry Potter series, Luna Lovegood.
As I considered Warners’ proposal, an idea took shape that I couldn’t dislodge. That is how I ended up pitching my own idea for a film to Warner Bros.Get excited!