Maybe one day, but not in the next expantion. Since 2007 we got an expansion with new races or a new combo race/class and the next with a new class. So, the next expack will be about new races and probably new combo race/class (subraces?), but no new class.
I. The Burning Crusade (2007) = new combo race/class + 2 new races
II. Wrath of the Lich King (2008) = DK
III. Cataclysm (2010) = 2 new races + new race/class combo
IV. Mists of Pandaria (2012) = Monk + pandaren
V. Warlords of Draenor (2014) = Classic Warcraft races updated
VI. Legion (2016) = DH + 1 new class race/combo
VII: XXXX (2018) = new races? subraces? new race/class combo?