Would you want to play an expansion pack that's based around WOW's version of the Underworld? Picture a continent accessible only through the permanent death of your character, but with the right anecdote, your character can venture forth into the abyss while still retain your flesh a as a living being. You would be transported to the spirit world, or the purgotary, where the spirits of your ancestor, fallen heroes and villains reside, before they may enter their true final resting place; WOW's version of heaven or hell.
There are couple of tidbits about WOW's after life, but it is still just a mystery as we only know only snippets of this either via stories or in game. Because the lack of lore, just like Pandaria, or any other new zones or continents previously had never been explored in depths before, an underworld expansion pack can be a breath of fresh air; where we can meet new villians, new lore characters, and at the same time meet with familiar heroes and villains with loose ends.
Where is WOW's spirit world, and how does it look like? Where is Arthas' soul being locked up and tormented. Can Sylvanas find a way to seek a final peace, because all she ever wanted after the death of Arthas, is a rest to end her suffering. Would the Ebon Blade consider to police this world if another evil such as the Lich King is born? What does the next world's society would be like? Who rules the underworld? And can a mortal enter it and make aa safe return into the land of the livings? What races or what new class would be made available if this expansion set happen?
There are also other aspects of WOW's lore that can be add in this via an underworld theme expansion pack; the religion of the Holy Light, Troll's Loa's , The Valkyrs, Old God, The legion of Seraphs and the fallen Seraph, Ibilis.