Think about it, if another leader takes the Lich Kings' place. and unlike the old one this new Lich King is on the living's side. we will have a third faction too.
a faction which includes undead warriors who watch over the living and make sure the dead stays where they are supposed to be. Northrend becomes their realm and Argent Crusade stays there to keep in touch with the new lich king.
this neutrality of the "new scourge" leads me to believe that in future (maybe WoW2 or warcraft 4 ) scourge will become a powerful "anti-villian" faction just like the Alliance and the Horde.
...now what entices me more is the thought of watching these 3 factions fight to "secure" a lasting peace .