Dark Ranger. The game needs another mail wearer and ranged weapon wielder. Combine that with both Sylvanas and Alleria being front-and-center this expansion and with the very likely possibility that 9.0 will have a Death theme, and Dark Ranger might as well have a neon sign above it flashing "Next Hero Class Here".
We'll know by Blizzcon 2019, so in a little over a year.