Here are my current views on what we could see in the next expansion. Feel free to add to the list.
New class skills/spells/talents
Scalable character level
Demon hunter class
New solo content (Brawlers 2 and possibly solo instances based of how they did proving grounds).
Pet battle instances
More pet levels
More competitive PvE (some examples - raid races, boss gauntlet, "horde" mode)
4th (5th for druids) specs.
New role - controller
Path of the titans