Honestly the only future class I see that doesn't trample all over existing classes is the Tinker. The Tinker has a pretty long history in the Warcraft franchise, and it has established lore in both opposing factions. There was even a Tinker hero in WC3 just like Death Knights and Demon Hunters.
I wouldn't mind seeing Goblins and Gnomes getting some love after the Elves got Demon Hunters.