I know, that's my other possible scenario, where they focus on the Second War and Beyond the Dark Portal more. Having Orgrim, the tragic antihero, as the Horde protagonist.
All of that was my "solution" to the posibility of them wanting to focus on Thrall and showing the foundation of Orgrimmar. My point is that they don't need the Third War or Arthas for it to happen, particularly if they have no intention of actually getting to those at all.
I'm pretty sure Warcraft is heavily focused on Lothar, Garona and Durotan, as the three main points of view of the First War.