The Tranquil Forest

If Khadgar isn't there for you to complete it, but instead shows up a little closer to entrance on Krasus' Landing, it's ok.

Talk to the Khadgar that is right there for you. You need to accept and complete the quest Khadgar offers on Krasus' landing called Armies of Legionfall, complete it right there, almost immediately. Then accept Assault on Broken Shore. You've likely already completed this on another character so you should be able to skip it, otherwise, just follow through and once you complete the quest, you can hearth back to Dalaran and Khadgar will be available for you to complete The Tranquil Forest. :) Hope this helps.

If you haven't unlocked Broken Shore, it seems you cannot complete this quest.

From what I can tell, it's possible to travel to Val'Sharah directly and leave this hanging. Malfurion's quest chain (about Cenarius) is an INDEPENDENT one, this is more like a breadcrumb quest leading you into the zone. I hate this story mess and inconsistency, playing the assault before the player even sets the foot on the islands and\or plays the class chain makes no sense at all.