Well, a new class should fit into the theme of an expansion, or at least some aspects of it. If we are to deeper explore the Void under Alleria's guidance, then a "Void Hunter" is possible. If gnomes and goblins are to play any significant role in the story for whatever reason, then it's Tinker. If they continue to develop Bolvar's story, then Necromancer isn't all that unlikely. Etc etc. The point is, specs and roles and mechanics are secondary, the first thing they look at is the theme.