So you want Blizzard to literally hold your hand and spoon-feed you every single available plotline in the game? Sorry, no company will ever do that, as they [rightfully] assume that the average player has some degree of autonomy.
You were told in this thread that Calia already played a role in Legion, in the Priest questline. Go make a Priest and play the questline related to Calia. It's that simple.
Also, it's not a "random questline". It's literally the main questline of the Priest. You do know that Legion was advertised as having massive class content, as each class had an entire questline tailored around them, right? It's not some obscure piece of story in that one forgotten zone of the old world, it's literally one of the main features of Legion -- Class Order Halls.