I could see a Burning Legion xpac going like this:
Combined Alliance/Horde command: "We are losing this war to the Burning Legion"
3rd Party: "There is a force strong enough to turn the tide out there"
Combined Alliance/Horde command: "Who?"
3rd Party: "Him, The Lich King"
Combined Alliance/Horde command: "Are you insane?"
Later on as we lose another major battle against the Burning Legion, the combined Alliance/Horde command approaches us adventurers "We need you to go to Northrend to the Icecrown Citadel and awaken the Lich King and convince him to join our side, otherwise all is lost."