So. The book has been out for two weeks now.
By the time Book Four comes out, we'll all be a good deal older. I'll probably have graduated high school. But I'm going to make some predictions anyways. Bear with me, folks.
I think the next book will be Szeth's, just because of the way his storyline progressed in Oathbringer. Perhaps it'll even be titled Stones Unhallowed. In the next book, Szeth will take up his crusade and return to his homeland, likely with Lift in tow. We'll finally see his spren, and their bond will grow. I think he'll meet back up with his family (That should be interesting. I wonder what Neturo is like?) and the people who made him Truthless, while the flashbacks show us how that whole ordeal went down in the first place. We'll see the Shin culture, the honorblades, and the Stone Shamans. I'm fairly certain we'll see Zahel and Azure again, since Szeth has Nightblood and that's what Azure is looking for. Maybe we'll learn more about why they came to Roshar. Maybe we'll even learn a bit about why humanity came to Roshar, seeing as Shinovar was their original territory on Roshar.
I think that, as The Way of Kings was a book about challenges, Words of Radiance was a book about growth, and Oathbringer was a book about passion, this will be a book about origins.
These are, however, just the thoughts and ramblings of a madman. And we've got at least 3 years until we find out anything, so who knows if these'll be anywhere near correct? Either way, feel free to give me your ideas, your arguments, your own predictions... anything. We've got plenty of time.