If Blizz wants, they could merge them all into 1 expansion, look how tight the story in the frozen throne is. Of course there is a bit less level of detail in a rts game than mmo game.
good one on the Sylvanas story, she is the true heir of the scourge. She and Bolvarlichking must deal it out sometime.
Also might add in that burning legion add on Kil'Jaden don't really need to be the main boss. Obviously it makes a lot of sense, but he is not the true leader of the burning legion, we should fight Sargeras. He should simply return. The Thunderking returned too, why shouldn't Sargeras? And fighting him doesn't mean we will win or deal much dmg to him.
But it can be a great raid fight and there can be loot for the winner.
Problem with Kil'jaden beeing a main antoganist is, he was the main boss in burning crusade allready. They wanted illidan but fact is, KJ made it. If we have to fight him again without twists as endboss that might the apocalypse of all minor setbacks.