Only for a time.
The death of Bolvar, being like a brother to the King Varian Wrynn - Forced the King to travel to Undercity, to destroy the Apothecary and the Dread Lord whom betrayed the Forsaken and killed one of his last people that stood by him for all that time.
Thrall, Voljin, and Sylvanas are all there; Varian tries to kill them all.
So after Lich King, it'll be good ol' Horde vs Alliance again.

It's a quest you get in Northrend. For Horde you go back and reclaim UnderCity; For Alliance you go to UnderCity to kill the Apothecary and the Dread Lord, then the King tries to kill the leaders of the Horde before Jaina Proudmoore freezes them all and teleports them back to Stormwind.
Yay for being nerdy about lore.