Just recently, Greyseer (the author behind Lorecrafted), finished up a three part speculation series on what he would like to see come out of a Maelstrom expansion. Reading over it, you realize just how many different forces could be at play in the expansion. I don't think everything will play out the way that he thinks they'll play out, but that just lends more weight to just how much could happen in the Maelstrom.
There are plenty of people that think Warcraft dies with Arthas, but when you look at the game world beyond the events of Warcraft III, Arthas is actually relatively small character. He's not even the most dangerous thing Azeroth has seen. The threats that lie within the Maelstrom have proved themselves to be far more destructive... as in, caused Azeroth to sunder into the various pieces that make up the world today. Let's see the Lich King do that.