I guess Necromancer and Demon Hunter will be the next 2 ones.
Necromancer because it fits very well in the story, and makes a pair with Sorceress as INT users/arcane oriented character. If the Necromancers are introduced to game, Hatma maybe will be a central part of events too.
Demon hunter as they showed bows, and crosbows in panel, but no one of the 3 previous class fits well with them to be a main ranged character. As they will put a great effort to show and explain Demon huters' creation on Diablo Imortal, i think they will put them on D4. And they fit well in story too.
BUT, i realy want to see WD coming back at D4. I never was a great fan of them, but i guess they were not well introduced in D3. Their relations to Hawezar region and Kurast city would bring a nice chance to make them shine at D4.