So if you check the Chronicles Warcraft Universe structure it has Light, Shadow, Life, Order, Disorder and Death. For Light its the Prime Naarus, Shadow is the Void Lords, Life is Elune, Order is Aman'thul and Disorder is Sargeras, but for that Death thing we dont know.
I dont think its the Lich King because he is just in Azeroth and can't represent death in the entire universe. Also when Arthas died he said ''i see only darkness'' and its mentioned that he has gone to ''Hell'' also known as ''The Dark Below'' which is its own realm in the univese so that means that there is a far stronger entity than the Lich King that is like the Grim Reaper/Angel of Death and represents death alltogether. This type of hell is most likely not the demons/fire fantasy because we already have that in the Burning Legion/Disorder which is a different thing, its more like a shadowy realm with different type of daemons/undead/lost souls etc. http://wowwiki.wikia.com/wiki/Hell/http://wow.gamepedia.com/Dark_Below
This also means that we could get another expansion focused on Lich King/Arthas in that Dark Below place redeeming himself and taking the fight to whoever is the leader of the Death realm